Step 5: Sand off the extra primer. (Only for primed timber)
First up, make sure you're paint is completely dry, and then take your pieces outside or anywhere away from where you're going to glue together, because trust me, that dust will get everywhere .
Now that you've done that, lay down a sheet of sand paper (I wouldn't get any coarser than 80 grit because you'll get a really rough surface) on a table or bench and start sanding off the primer on the unpainted sides of your squares; you may want to leave a side un-sanded on the edge pieces to make painting them easier once the whole thing is glued.
You do not need to get the primer completely off, just a good rub to get most of it off will do; keep in mind, if you sand too much, your pieces won't fit right. Also, make sure you have at least 3 sheets of sand paper, it wears quickly.