Making sure that each joint was perfectly square, in turn we pre-drilled each screw hole. Some sanding may be necessary... so I HIGHLY recommend screwing each cabinet together to ensure a proper fit BEFORE actually gluing it permanently.

When you're sure that it's all going to fit nicely: glue, screw, and clamp all of the sides except the back. Once you attach the crossovers -- go ahead and affix the rear as well.

The glue should form an incredibly powerful bond. In fact, you can go ahead and remove the screws once the glue has set completely. If you feel more comfortable with leaving them there, just be sure to countersink.