Make sure you gather some nicely shaped leaves. Dry them if needed, place a some heavy stuff (like a glass) on the pressure points of the leaf to prevent it from curling up on the sides.
Choose the colors you want your leaf print to be, and start painting the leaf.
After painting the leaf, try to make the print as soon as possible. This to prevent the paint from drying. When you put your leaf on the canvas, make sure you gently pressure every part of the leaf.
Remove the leaf gently and voilà, your print is ready. Let it dry for a couple of hours and enjoy your painting. (P.S. It's also fun to make a print on a tote bag, but use textile paint).
You can let the leaf dry as well, it's fun to hang it on the wall or to make a garland out of it.