Rinse the river rocks to remove any dust and allow to dry completely.
Paint rocks orange and allow to dry. You can paint both sides or just the side you plan to decorate. You can use acrylic craft paint and a foam paintbrush, although if you are making lots of rocks spray paint with be faster. Two coats may be needed.
If desired, use the ultrafine Sharpie to add curved lines down the front of the pumpkin. See note for tip about using circle templates here.
To decorate the pumpkin rocks, start by using the green paint pen to draw a stem near the top of the rock. Add curled vines if desired.
Use the black paint pen to draw a Jack-o-lantern face on the pumpkin. Search online for jack-o-lantern clip art to get ideas.
When all the paint is fully dry, seal the pumpkin rocks with a clear spray.