You paint them just like you would if you were chalk painting a piece of furniture. Paint them and let them dry. Here I used Paris Grey and French Linen. Then create a wash.
Mix a small amount of white chalk paint and water together for your wash. I used about 2 tsp of paint and then added 2 tsp of water. You want this to be thin but also thick enough that it holds.
White washed pumpkins are very pretty. The wash adds just a little different twist to your other pumpkins. Let me know if you have more questions.