How do I make a double underline?

Unless this is something you need to do with some degree of frequency there is absolutely no need to clutter up a toolbar with extra buttons. Even if it needs to be applied with any degree of repetition it would be more practical to create a Character Style to include the dbl underline attribute.

For a one shot instance just select the characters to be underlined then go to Format> Font & select double from the list of Underline Styles.

Another option: If the column of figures is in a Table (even a single column just for the figures), remove the Borders from the Table & apply just a double line Bottom  border to the cell containing the final figure. IMO this makes for a much more "accounting friendly" appearance because the border isn't as close to the number. It also enables adjusting the amount of separation in several different ways, whereas Underlining is fixed.

Even more options are available if you create & format the list of figures in Excel, then copy/paste special into the Word document.

See this Movie:

http://www.screencast.com/t/qpTo8PKoHCJq

when you get to desired button click on it and Drag to desired location on Formatting Bar.

Then click okay and when you save document and Quit word. You may be asked if you want to keep changes in Normal.dotm. choose yes.

IF you don't receive the request then next time you open word go to Word menu > Preferences > Save, and click check box next to Prompt to save Changes to Normal.

That way it will ask if you want to to keep the changes you added.  IN this case it would be yes. Sometimes you add thing temporarily for just a given Document. In that case you would answer No

IF you add something mistakenly be it buttons, or command for the menu bars, just drag them off of the menu bar or the toolbars and they will disappear in a cloud of smoke.