Measure the interior of your window where you’d like the cover to sit. For the height, be sure to measure a bit generously from the top of the AC unit; for the length, subtract a half inch or so to ensure your cover will fit snugly within the window. (I made my frame to the exact measurement the first time and it didn’t fit.)
An important note: We have deep windowsills, so I opted for narrow strips of wood that would stand up on their own, like a frame, in front of the AC unit, and still leave quite a bit of bare windowsill. If you have shallower sills, and your unit overhangs them into your living space, retrofit this design with wider pieces of wood, so that you end up with less of a frame and more of a deep box that can fit snugly over your unit. (The bottom can fit snugly between unit and sill.)
2. Make a frame.
Cut your four pieces of wood to size: two strips for the length you measured, two strips for the height. The wood that I got from the art supply store was thin enough that I cut it with an X-Acto knife (I wanted my frame to be extra lightweight), but you could also use hardier wood and have it cut to your measurements.
Also note: If you find a ready-made frame that fits your measurements, even better. I made my own because I wanted it to fit perfectly, and no frame I could find was just right.
Glue the four lengths of wood into a rectangular frame using wood glue, one corner at a time, using something with a right angle (like a notepad) to ensure that all of the corners are square. Hold each corner in place as it dries.
Leave the frame for at least half an hour to let the glue harden. Then, if the wood is thick enough, you can add some hardware to secure it if need be. Mine was fairly sturdy, and the wood was thin, so I added triangular supports in each corner, cut from spare pieces of wood and adhered with wood glue, instead.