I have seen this activity before but I can't remember where.
You just cut an apple in half, dip it in paint (or paint it with a paintbrush) and then use it as a stamp. We used them to make pumpkins.
We did the stamping in the evening, and since they used a lot of paint,we let them dry overnight before adding googly eyes & paper jack-o-lantern faces. (I cut out the pieces & Talia arranged them).