Step 2: Modification
The main point of this step is to build a frame around the A/C unit that will fit inside of your window frame. If you chose to use the plastic that came with the A/C follow these steps. However I can't take responsibility for your installation, theft, break ins, and your A/C falling out of your window.
Do not forget to make the 5 degree tilt on the frame for proper drainage.
You could also use the C-channel or square tubing (both available at Home Depot) to build a frame around the A/C so it will fit inside of your window frame.
In the installation manual the first thing they ask you to do is to mount the accordion looking curtains.you should throw them away.
The inside of the plastic is slightly rounded on the inside of all corners, use a saw, or cutters to give it a nice 90 degree angle.
Before you mount your plastic around the A/C, break off the metal on the bottom of unit, or bend it out of the way. It's not needed, it will not fit in to the window rail.
Your frame has to be well attached to the A/C. Use more screws if in doubt.