Step 1: Let your 8yo pick the font, within reason. I only vetoed two of his picks for readability issues, I'm growing here. Using columns to conserve paper, copy and paste, "Whoopee, I'm glad we're friends! Love, the Nugget." Trim with paper cutter.
Step 2: Give your 8yo a whoopee cushion to test. Watch him casually invite Daddy to have a seat on a suspicious napkin and roll on the floor in hysterics. Yes, this will all be worth it.
Step 3: Place a whoopee cushion and the cardstock in a cello bag. Close with washi tape of your 8yo's choice.