Step 4: Finishing the Frame
To figure out the placement of this piece of 2X4, lean the A/C so that the front vertical piece of the top lip lines up with the front edge of the wood frame. This is how the A/C will be oriented in the frame when complete, tipping the unit back.
While you're holding the A/C in place, take a look at the whole configuration to make sure it leans back sufficiently. Air conditioners must drain and are built to tip away from the window. If this lean appears to be too little (it really shouldn't be if the top and bottom lip are properly aligned with the frame), then you might want to rip this last piece of lumber, making it less than 3.5 wide, before screwing it in place.
If everything looks o.k., lay this piece of lumber in place and mark its position on the frame. Also, mark the top edge of the front lip where it will hit this piece of framing. (This mark is so you can measure for the piece of plywood covering the hole in the top of the frame.)
Remove the air conditioner from the frame and screw this piece of lumber in place. It would be good, at this point, to dry-fit the A/C into the frame just to make sure you are on-track.