The curious nature of cats will make it easy for them to figure out, but follow these steps for the best results:
Place your new Modkat litter box in the same location as your old litter box.
Fill the liner with 3-4 inches of litter, adding at least one scoop of litter from their old box.
Once they smell the familiar scent they should know what to do.
If they don't seem to get it, try the following:
If they won't go near their new litter box, gently place them on top, and let them jump off or out themselves. This may help them to become familiar with it.
Reintroduce their old litter box, and place their new Modkat next to it with the lid off.
After three days replace the Modkat lid.
On the fourth day take away the old litter box and see how it goes.