Step 4: Start attaching those shells!
Start by layering your shells, biggest on the bottom, smallest on top. Make sure you overlap where the foam attaches to the cube. Be generous with the glue, any that squishes out can be removed with a paring knife / Xacto knife.
A little tip to hide glue bulges in the joints where shells meet is to use tiny shells on top.
To apply shells, I found it easiest to find the broadest, flattest portion of the shell and glue that part down first. Since the glue stays soft for a few minutes, you can then see where the rest of the shell touches the foam / cube, gently lift it up, pump a little glue under it, and press it back down.
Use long, skinny shells or large, broad ones to support smaller ones. Add plenty of depth. Don't limit yourself to just the foam corner, that's just where the focal point of each cube will be.