Hello,
There are two easy ways to do this, with the first option using Control Panel, the second option using WinRE.
The first option is to go to the Control Panel, and then once Control Panel opens, immediately under System and Security is "Backup and Restore (Windows 7)". Click that, and then under "Restore", the WIM file that was backed up to your external hard drive should be there. If it isn't, push "Select another backup to restore files from", and if prompted by UAC, push yes, and then select the WIM file that you backed up to your computer.
The second option is to use WinRE. To go to WinRE, simply push the power button in the Start Menu, and push Restart while pushing the Shift key. If this was executed correctly, there should be spinning dots that say "Please wait" below it. Once WinRE pops up, push Troubleshoot, then "See more options", then push the option that says, "System Image Recovery". Once the dialog box pops up, select the WIM file that you backed your stuff onto (if it's not there already), and then follow the prompts there.
Let me know how this goes and if you have any questions, don't hesitate to ask!