It's important to allow your sublimation puzzle to cool completely before adding your sublimation print to avoid any ghosting.
Place your sublimation puzzle, white side down, onto the sublimation print making sure it's located how you want it.
If you're not sure which side to sublimate on, the sublimation side should feel smooth and look shiny. The back has a rough texture and is off-white in color.
This is where we need that cardboard piece. Adding tape directly to the back of the sublimation puzzle will damage the puzzle itself.
With your puzzle located on the print add the cardboard over the back of the puzzle and then tape the cardboard to the print. You want your sublimation puzzle sandwiched between the cardboard and the sublimation print. This way the tape will ruin the cardboard and not the puzzle.
You can see that I have taped my image into place on all four sides of the puzzle with the tape on the cardboard.