Print your water soluble paper inserts and spray them with an extra-fine mist hairspray to set the ink. You should let the inserts “cure” for several days before use.
When pouring the first layer, let the soap cool down some before pouring and then press the inserts in place when the soap begins to form a “skin”. Since the paper inserts are water-soluble, you should not spray rubbing alcohol between the two layers.
Instead, let the first layer sit and cool for almost an hour before pouring the second layer. When pouring the second layer remember to let the soap cool down some before pouring it.
We discovered that by waiting and letting the soap cool down before pouring, the ink did not run and the soap remained clear or white instead of showing evidence of yellowing. Higher temps always resulted in the ink smearing and discoloring the bars. The lower temperatures produced extremely clear soap plus the bars seem to hold together better (meaning not coming apart at the layers).