First, decide what you are using for your top. I have been using two 2x6's, which make mine about 18"x11" (these shown area little smaller--they were a special order to fit under one of the tables I had made).
Then cut 2 pieces of trim board or 2"x4"s 2 inches shorter than the length of your top.
I set these on the top and then measure what I need for the shorter side pieces. I have not been doing mitered corners because I use trim boards and it is too thin. I assemble the apron or frame with the nail gun. Since I screw the legs in from both sides, this doesn't need to be solid at this point.
Lay frame upside down on a flat and level surface. I screw legs in with 2 screws in the front and one 3 inch long screw from the side.