How to boot Windows 7 Professional from USB?

Restart your computer and wait for that first screen to pop up. Often, it'll say something like "Press F12 to Choose Boot Device" somewhere on the screen. Press that key now.

I don't think you can actually boot Windows 7 from a flash drive. You can, however, boot the installer by creating a bootable flash drive using the Windows USB/DVD Download tool: https://www.microsoft.com/en-us/download/windows-usb-dvd-download-tool