The Layens frame assembly jig is a simple jig to make and gives back many times over in it's usefulness. It helps to ensure that the frames are assembled square and also securely. If you are going to assemble many frames at all it is worth taking the time to build one of your own.
The parts are as follows: 1 base, 4 side rails and 3 spacers.
The base is made from 3/4" thick plywood. Cut this piece to a size of 12-15/16" wide X 15-15/16" long.
Next, cut 4 plywood rails (they can be any thickness, I used 1/2") into 2" X12" pieces.
Lastly, make your spacers. Cut 2 (from 2X material) at 3/16" thick X 12" long. Cut one (from 2X material as well) at 5/16" thick X 12".
Assemble jig by first fastening the 4 rails to the sides of the base. I glue and nail these in place. Stand them on edge ( 1/2" side facing upward) in the center of each side and flush with the back edge of the base. Next glue and fasten the 3/16" thick spacers along the rails on each side and the 5/16" thick spacer along the rail at the top. And that is it. Give the glue time to set up and you jig is ready to use.
To assemble a frame in the jig, first place a top bar on edge along the bottom rail 
(the one with no spacer), then a frame side bar along each side rail and lastly a bottom bar between the two side bars along the top rail. I use two 1" brad nails or one 1" narrow crown staple in each joint. Too many fasteners will weaken the wood so don't get carried away with the nail gun.