Cut the block of soap base into cubes along the perforated lines. Place 4 cubes in a microwave safe dish and microwave for 20 seconds. Stir. Repeat in 10 second intervals until melted.
Add 6-8 drops of soap scent oils. I have a box with 10 fruity scents and they all smell delicious: pineapple, mango, strawberry, watermelon, apple, cherry, cucumber melon, lemon, orange, and peppermint (obviously not fruity, so that one was a surprise!) These work great and can be used for homemade lip gloss, too.
Place soap mold onto a small baking sheet. Pour melted soap into a silicone mold. Allow to set in the refrigerator 30-45 minutes.
Cut out desired temporary tattoo.
Place tattoo onto soap.
Apply a wet sponge or wash cloth and hold for 30 seconds.
Peel away backing.
Note: to speed up setting, place mold on a baking sheet before adding soap. Then refrigerate for 15 minutes.