Step 4: Glue em together
I used "Resintite" wood glue. Just use plenty of clamps and keep checking everything's square.
For the more complicated shapes you'll want to do the gluing in a few phases. Maybe make a jig to help you along too.
You'll have a little bit of play in the joints before you put the back panels on, but it can't hurt to get em as accurate as possible now.
I scraped the glue on the inside joints off now, as sanding it off later would have been a nightmare.