Once you have the concept down, you'll need to go shopping for materials. We found the kind of storage cabinet we wanted for this project, took measurements at the store, then went hunting for a plastic container to serve as a litter tray. An inverted wire storage drawer served as an entrance/exit deck. We also needed four 1" L-brackets.
Step 1: Measure and cut ingress / egress
Step 2: Extra hardware
Step 3: The litter canopy cage
Use cutters to snip away cage wires where you've marked. Be sure to carefully file/grind any edges!
Step 4: Final assembly
Be sure the litter tray can easily slide out for scooping and cleaning.
Hmmm. Looking at this photo, I may yet add another shelf at the entrance for the cats.