I bought some jeans at the Goodwill for $5, picked out a fun color and got to work!
Fabric Paint (I got this at Michaels)
Color Paint (also from Michaels)
Step 1: Make sure the jeans are washed!
Step 2: Mix 2 parts paint with 1 part fabric paint
Step 3: Apply an even coat to the circle sponge
Step 4: Start making polka dots
Step 5: Allow the front to dry and then flip and repeat for the back
Step 6: Rock your dots
Side note: I did find I had to do about 2-3 coats of the dots, to give them the pop of color I really wanted them to have.