Spread condensed milk on a wafer disc, completely covering it to the edges. Spread condensed milk only on the top side of each wafer disk.
Place another wafer on top of the one that is covered with condensed milk and repeat the same instruction by stacking the disks of the different colors until you use up the entire pack.
Place the cake under a press for an hour or so. I place a cutting board on top of the cake and place a few pounds of weight, like a jar of pickles, on top of the cutting board. This step will keep the cake layers compressed evenly while they are absorbing and softening from the condensed milk.