I haven't done it from USB. Nor have I put windows installer on USB. All my windows installs (XP, Vista, 7) are done with CD/DVD but process is same. Only USB installer I've done was for Ubuntu but again, same process.
Here's the Linus how-to video:
Here are dl links for Win7 ISOs. If you have Win8 or 8.1, use Microsofts own service.
http://www.techverse.net/download-windows-7-iso-x86-x64-microsofts-official-servers/
Download the .iso, get windows USB installer tool, find 8gb (or any 4gb+) USB stick and format it. Use the tool's wizard. Go into BIOS and select USB to be 1st boot device. Boot into installer and follow this guide http://www.sevenforums.com/tutorials/1649-clean-install-windows-7-a.html
The guide there is really good so you should be fine. Oh, and here's the WIndows USB tool as bonus http://www.microsoft.com/en-us/download/windows-usb-dvd-download-tool
