Cover all the parts you don't want painted red (or any color you want to use in the top of the mushroom for that matter) using masking tape and stickers with the shapes you want. In the mushroom's case I used round stickers to make the dots on its top.
Use the red spray, don't point the can to the object from up close, do it from a distance in thin layers, wait a while in between each coating. You don't want the paint to over accumulate or drip... Uncover the masked parts.
Use a black permanent marker to paint the eyes and there you have it! Mushroom clone #1!