In addition to the patterned tape I found in the post below, I also purchased this package of cute erasers. What a big spender! These are perfect for embedding in soap because they're not too thick.
I poured in a thin layer of pink coloured goats milk soap and let it harden a bit, you want it to be thick enough that you can blow on the soap and not have it move, then you know it can take the weight of the eraser. I then put a little clear melted soap on the back of an eraser and gently set it in the mold. I don't think erasers float but you never know :) Then I sprayed the soap and eraser in the mold with rubbing alcohol and filled the mold the rest of the way with clear soap.
Muffin tins , little tupperware, or silicone molds would be great for a project like this. These eraser packs would be worth buying and putting away for a future birthday or sleepover party project. It doesn't have to be February to love hearts!