Sorry for the vague post (and not pictures), but that was on purpose.

First off, let’s insert your style sheet part to the workbook (check out how to create a workbook):

WorkbookStylesPart stylesPart = spreadSheet.WorkbookPart.AddNewPart<WorkbookStylesPart>();
stylesPart.Stylesheet = GenerateStyleSheet();
stylesPart.Stylesheet.Save();

Now to implement the GenerateStyleSheet method:

private Stylesheet GenerateStyleSheet()
{
    return new Stylesheet(
        new Fonts(
            new Font(                                                               // Index 0 - The default font.
                new FontSize(){ Val = 11 },
                new Color(){ Rgb = new HexBinaryValue() { Value = "000000" } },
                new FontName(){ Val = "Calibri" }),
            new Font(                                                               // Index 1 - The bold font.
                new Bold(),
                new FontSize(){ Val = 11 },
                new Color(){ Rgb = new HexBinaryValue() { Value = "000000" } },
                new FontName(){ Val = "Calibri" }),
            new Font(                                                               // Index 2 - The Italic font.
                new Italic(),
                new FontSize(){ Val = 11 },
                new Color(){ Rgb = new HexBinaryValue() { Value = "000000" } },
                new FontName(){ Val = "Calibri" }),
            new Font(                                                               // Index 2 - The Times Roman font. with 16 size
                new FontSize(){ Val = 16 },
                new Color(){ Rgb = new HexBinaryValue() { Value = "000000" } },
                new FontName(){ Val = "Times New Roman" })
        ),
        new Fills(
            new Fill(                                                           // Index 0 - The default fill.
                new PatternFill(){ PatternType = PatternValues.None }),
            new Fill(                                                           // Index 1 - The default fill of gray 125 (required)
                new PatternFill(){ PatternType = PatternValues.Gray125}),     
            new Fill(                                                           // Index 2 - The yellow fill.
                new PatternFill(
                    new ForegroundColor(){ Rgb = new HexBinaryValue() { Value = "FFFFFF00"} }
                ){ PatternType = PatternValues.Solid })
        ),
        new Borders(
            new Border(                                                         // Index 0 - The default border.
                new LeftBorder(),
                new RightBorder(),
                new TopBorder(),
                new BottomBorder(),
                new DiagonalBorder()),
            new Border(                                                         // Index 1 - Applies a Left, Right, Top, Bottom border to a cell
                new LeftBorder(
                    new Color(){ Auto = true }
                ){ Style = BorderStyleValues.Thin },
                new RightBorder(
                    new Color(){ Auto = true }
                ){ Style = BorderStyleValues.Thin },
                new TopBorder(
                    new Color(){ Auto = true }
                ){ Style = BorderStyleValues.Thin },
                new BottomBorder(
                    new Color(){ Auto = true }
                ){ Style = BorderStyleValues.Thin },
                new DiagonalBorder())
        ),
        new CellFormats(
            new CellFormat(){ FontId = 0, FillId = 0, BorderId = 0},                          // Index 0 - The default cell style.  If a cell does not have a style index applied it will use this style combination instead
            new CellFormat(){ FontId = 1, FillId = 0, BorderId = 0, ApplyFont = true },       // Index 1 - Bold 
            new CellFormat(){ FontId = 2, FillId = 0, BorderId = 0, ApplyFont = true },       // Index 2 - Italic
            new CellFormat(){ FontId = 3, FillId = 0, BorderId = 0, ApplyFont = true },       // Index 3 - Times Roman
            new CellFormat(){ FontId = 0, FillId = 2, BorderId = 0, ApplyFill = true },       // Index 4 - Yellow Fill
            new CellFormat(                                                                   // Index 5 - Alignment
                new Alignment(){ Horizontal = HorizontalAlignmentValues.Center, Vertical = VerticalAlignmentValues.Center }
            ){ FontId = 0, FillId = 0, BorderId = 0, ApplyAlignment = true },
            new CellFormat(){ FontId = 0, FillId = 0, BorderId = 1, ApplyBorder = true }      // Index 6 - Border
        )
    ); // return
}

Finally we apply the styles to the individual cells using StyleIndex:

new Cell(new CellValue("Bold")) { DataType = CellValues.String, StyleIndex = 1 },   // Cell A1

You should get the following:

For any developer with half a brain, knows that trying to write excel documents using the old Xml classes approach is just not feasible.  You have to explicitly write out each of the <tags> properly formatted.  There could be thousands of these nestled in some 5 levels of for loops with hard coded strings littered all over the code.  Yeah.... NO

Open XML 2.0 provided us with a little more abstraction with typed classes.  So instead of Xml-ing <Cell>, you would create a Cell object.

So now lets try and create an Excel document from scratch using Open XML 2.0 (April 2008 CTP) in C#.  I am going to assume you have some knowledge the OpenXML specification.  If you are reading this cold turkey, stop now, go learn about the spec.  Theres videos, no complaints.  www.openxmldeveloper.org

Have a look at a basic Excel 2007 spreadsheet by creating one in excel.  Make sure to delete the 2 other sheets below and put some value into the A1 cell, say 100.  Save the file, then rename the extension to .zip and have a look around.  Specifically jump into the xl folder and open up all the .xml files in it. 
workbook.xml holds all the sheets info and the sheet.xml in the worksheet folder holds the actual data.  the rels file is whats links the worksheet to the workbook.  There are more stuff but you can play with that on your own.

Programming:

Just to make this a little easier, I am going to reference everything using the full object path.  I know sometimes working with object references can get confusing.  It is nice to know exactly where you are and what you are referencing without having to hold it in your head. The heirarchy we need to follow is simple.

• Create Spreadsheet
• Create Workbook
• Create Worksheet
• Create SheetData
• Create Row
• Create Cell
• Link Worksheet to Workbook
• Enjoy Ice cream

using (SpreadsheetDocument spreadSheet = SpreadsheetDocument.Create(
    System.IO.Path.Combine(AppDomain.CurrentDomain.BaseDirectory, SPREADSHEET_NAME),
    SpreadsheetDocumentType.Workbook))
{
    // create the workbook
    spreadSheet.AddWorkbookPart();
    spreadSheet.WorkbookPart.Workbook = new Workbook ();     // create the worksheet
    spreadSheet.WorkbookPart.AddNewPart<WorksheetPart>();
    spreadSheet.WorkbookPart.WorksheetParts.First().Worksheet = new Worksheet();

    // create sheet data
    spreadSheet.WorkbookPart.WorksheetParts.First().Worksheet.AppendChild(new SheetData());

    // create row
    spreadSheet.WorkbookPart.WorksheetParts.First().Worksheet.First().AppendChild(new Row());

    // create cell with data
    spreadSheet.WorkbookPart.WorksheetParts.First().Worksheet.First().First().AppendChild(          
          new Cell() { CellValue = new CellValue("101") });

    // save worksheet
    spreadSheet.WorkbookPart.WorksheetParts.First().Worksheet.Save();

    // create the worksheet to workbook relation
    spreadSheet.WorkbookPart.Workbook.AppendChild(new Sheets());
    spreadSheet.WorkbookPart.Workbook.GetFirstChild<Sheets>().AppendChild(new Sheet()
        {
            Id = spreadSheet.WorkbookPart.GetIdOfPart(spreadSheet.WorkbookPart.WorksheetParts.First()),
            SheetId = 1,
            Name = "test"
        });

    spreadSheet.WorkbookPart.Workbook.Save();
}

Much like Xml usage, we navigate the spreadsheet document by calling it's children.  You can append the child elements in any order.  As long as it is properly id/referenced it should show up in the correct spot.  In this example I knew that the first element being returned is exactly what I wanted since I placed it there.  For your automated system you may want to add checks or use the type system to sort out child elements you really want to manipulate.

    spreadSheet.WorkbookPart.WorksheetParts.First<WorksheetPart>().Worksheet = new Worksheet();

Better yet, use Linq.

 // edit: fixed a spelling error

<TextBox>
    <TextBox.Text>
      <Binding RelativeSource="{RelativeSource Self}" Path="Text" UpdateSourceTrigger="PropertyChanged" />
    </TextBox.Text>
</TextBox>

Once you type something in the text box, whammo!  StackOverflowException.  See: https://forums.msdn.microsoft.com/en-us/wpf/thread/bf300eea-5856-4738-b565-b8e2f5e52195/ for reasons why.

Workaround:
The easiest way to get around this is to set the UpdateSourceTrigger to "LostFocus,"  the default setting.  However, this leads into some hackery causing you to force validation by focusing on the control property. 

textBox1.Focus();
someOtherControl.Focus();

If you really want a validation on every character entered, you can approach this is by creating a private string dependancy property, and using a border to simulate the red outlining.

public static readonly DependencyProperty MyStringProperty = DependencyProperty.Register(
     "MyString",
      typeof(String),
     typeof(Window1));

public String MyString
{
     get { return (String ).GetValue(Window1.MyStringProperty ); }
     set { SetValue(Window1.MyStringProperty , value); }
}

public Window1()
{
     InitializeComponent();
     textBox1.TextChanged += delegate(object sender, RoutedEventArgs e)
     {
          if (Valid())
          {
               border.BorderBrush = null;
          }
          else
          {
               border.BorderBrush = Brushes.Red;
          }
     };
}