2. Use a drill to make a hole next to the line (inside the circle) big enough for the saw. Then insert the saw into the hole and cut out the circle.
3. Use sandpaper to sand the circle and get rid of any possible splinters.
4. Assemble the front to the rest of the bench.
Optionally, you can use paint to draw something around the hole. It is certainly not necessary, but it helps hide any possible unevenness. (I recommend using a primer first). I also added some adhesive hooks for the shovel and bags.
I bought the STUVA second hand for 30€ (not sure what it used to cost since it is not in the catalog anymore). The SMÅSTAD bench 90x52x48 (69€ now) is pretty similar.
What to pay special attention to?
If you use the drill too close to the line you may end up with an uneven circle. Also, make sure the cat litter box will fit inside the STUVA bench, because I only thought about the outside measurements and my litter box ended up being only a few centimeters short of not fitting in.