Step 12: Drill pilot holes and put in screws
After glueing, you'll need to strengthen these connections with screws. In this main pic, I have clamped my wood together and am measuring my countersink drill bit for the pilot hole. I don't want the bit to go all the way through, just about an inch and and a quarter, to match the length of my screws. After measuring, drill the pilot hole. If you don't have the countersink drillbit like me, you can use a plain countersinker bit, or even just use a large drill bit to make the divet for the screw head to be flush.
Now change to the philip's head screw driver bit and put your first screw in.
Then, repeat process - pilot hole drilling and screwing - on the other side of the same stick of wood. Now you should have a U-shaped frame, open on one end. Don't attach the other piece of wood yet, read the next step first, before completing the rectangular frame! (Thanks to jwm.herbert, for this clarifying comment).