Dump a little paint (we used acrylic and craft paints) in the jar. Screw lid tightly. Let your kiddo shake it up. Shake it up. Shake it like crazy. Until the inside is coated with color. You'll want to remove the lid so the paint can dry or you'll just end up with a wet mess in there and a bunch of paint on the bottom of the jar (if the paint is runny). One of my paints was runnier, and I had to "roll" the paint around in there a few times while it was drying, so it didn't puddle the bottom.
We did a monster, a pumpkin, and a ghost. I painted the lids accordingly. Aiden picked some stickers to use for the ghost's eyes and monster's mouth, we added Googly eyes to the monster, and I drew a face on the pumpkin and a scar and bolts on the monster with a fat permanent marker. We had fun with this super simple idea I got from a fellow mom. Thank you!