Step 1: Go down the pub
Step 2: Get free postcards.
Step 3: Cut up postcards to fit your soap molds.
Step 4: Put postcard pieces face down in the bottom of the mold.
Step 5: Pour clear glycerin soap over the center of your postcard piece.
For the rectangular mold I've got, I thought using a scrapbooking corner rounder would help with the fit, but I have since learned that an exact fit means it won't work--you want room for the object to float so you get full coverage.
And even if you do have room, sometimes you still end up with fail-soap. But in my defense, trading card soap would be one of my most awesome, lazy craft ideas if I could actually get it to work.
Also, Hobby Lobby's brand of glycerin soap base is now on my shit list. It's cheaper than the brand Michael's carries, but that's probably because HL's brand turned out cloudy/murky.