1. Use a program to make your USB stick bootable (like you make a DVD bootable, caution, your USB stick is going to get formated, and you will loose all data on it, so back it up somewhere) I use wintobootic for windows 7 and 8.
2. After the program is finished making your USB stick bootable, stick the USB stick into your PC's USB port.
3. Restart your Computer, and while it is doing that hit del (or what ever gets you into your motherboards bios) over and over again, until you reach your bios.
4. Now search in the bios the place where you can choose what you want to boot from. Save and exit. The rest is like with a DVD.
5. When I do it with a USB stick, I stick out the USB when (it happens only ones) the 10 sec countown starts when you install the operating system. If you don't do it, it will repeat it all over again. So keep an eye on the installation process, or you have to do it again.
If you didn't understand something, just write.
Edit: with let it sticked it I meant stick it into your PC you want to install it.
I understand everything. Thanks for clearing that up for me!
Maybe you could help me answer another small question-
Are OS disks reusable? Say my friend buys a PC along with Win 7. He uses the disk first to install Win 7. Can I use it a second time on my PC to install Win 7? I've heard once an install disk is used, it is 'locked' to the motherboard it was installed on.
You can use someone else's install disk for a trial period with windows 7. You will eventually have to buy your own number. With Windows 8 this can be done as well but you need to do a few steps first such as loading to USB stick and installing a small file.