Materials Needed:
Raspberry Pi computer.
MicroSD Card and Reader.
HDMI Cable.
TV or HDMI Computer Monitor.
Ethernet Cable or Wifi Dongle (if Raspberry Pi 3, wifi is built in).
Android Power Cable with USB.
USB Keyboard and Mouse.
USB Game Controller of your choice.
Flash drive equal in size or lesser than the MicroSD

1. Go to the RetroPie website and download the image for your pi. It will either be "1" or "2/3"
This is a GZ file. Use 7-zip to extract it first, or you'll mount the wrong thing and like I accidently did once, confuse yourself.

2. Get Win32DiskImager software from sourceforge. Now open it.

3. Select the RetroPie image file and under "Hash" select MD5. Select the drive of your MicroSD Card slot and hit "Write" next.
NOTE: NEVER open this drive in windows or it will ask you to format. If you accidently do, immediately hit cancel.

4. Hook your Raspberry Pi up with the android cable, hook up HDMI between it and your choice of screen, and plug in the keyboard, mouse, and gamepad.

Remove your SD card from the PC, and stick it in the Pi. Power on.

5. Set up controller after first holding the 'A' button.

6. Grab a flash drive. I'm using a 64gig so I can fill it up twice and use it on my 128gig MicroSD. Create a folder called "retropie" with your PC

7. Stick it in the Pi and wait until it stops glowing. Now put it back in your PC and enter the retropie/roms folder. Fill these folders with whatever you want.

8. Stick it back in the Pi and wait again until it stops glowing. Restart the Pi by pulling out the android cable and putting it back in. You're done!