Now comes the fun part. The Kreg Jig I mentioned in the intro is invaluable to this instructable. The kit cost around $30 if memory serves correctly and is simple enough for a child to use (if you let children use power tools). The jig includes everything you need except for the clamp. It also includes the screws but you will probably need to buy more. They have a special head on them for use with the driver attachment included with the jig. I won't give a step by step on how to use it but the pictures are pretty self-explanatory.
When you construct the frame, its best not to do it on the floor as I did. The reason is because the clamp will raise up the pieces when you drill them together and you will end up with a frame that is warped.
ALSO NOT PICTURED BUT VERY IMPORTANT!!! Square up your sides with a carpenter square and check it after you screw the pieces together to keep it square!!