How does the foam reattach?

Plastic pins are inserted into the removed pieces, and plugged into the bottom layer.

Do I need to put a pin in every foam square?

No, just a few per block of foam you need to re-attach.

My case is a different size than the size listed, will it still work?

Yes, it will work with any case that has at least 3" internal height. The standard sheet size is 350mm x 550mm, so get as many as you need to fill your case.

What is the size of the foam squares?

Each foam square is 10mm.

Can I use this as a single layer in thinner cases?

Yes, however two layers are needed to be able to re-insert foam.