Step 1: Make sure you pick a day that's kind of rainy in the morning, but not too much so there's still sunshine to enjoy. Today, for example, was perfect. The sky opened up as I was driving to the Royal Botanical Gardens to meet my friends for a playdate, but by the time I got there it said "Just kidding, cue the sun!"
Step 2: Bring lots of children. Especially three-year-olds, and a 16-month-old who thinks she's a three-year-old. They will get each other nicely riled up. They may even roll on the ground for you!
Step 6: If they aren't dirty enough yet, FINGER PAINT!