For me, I get the Filtrate air conditioner filter at Walmart, (the one with the highest filter efficiency)
I took out my OEM filter, used a sharpie to outline the dimensions on the big filter, and cut out the filters with scissors.
To install I just "flare" out the width (to ensure the best fit possible, since mine doesn't have a tray I can tape to ensure a seal), and just slide it in there.
Always becomes discolored very quickly and fills up with dirt, which is why I change it out quite often. But for the cheap price of making your own, its worth the little work to make them!