Hey Oooooo.

What we have here is the same document that your prof provided in your actual workshop on making stretchers. There are two links. The first link is to the Illustration showing what is necessary to making a 30"x40" stretcher. The second link is to the written instructions of the process. 

Remember that unless you have been specifically trained/authorized to use the table saw (which usually is for intermediate painting students who have specifically requested the training) you must bring your 1"x4"s which have been cut to length for the shop technician to bevel the outside edges and then split your pieces so you can assemble your stretcher.

Take care in cutting your pieces to length and make sure they fit tightly together. The strength of the stretcher comes from the inside and outside frame fitting nicely together and then being glued and nailed. Give time for the glue to set up (8 hours) before stretching the canvas.

Larger stretchers can be significantly strengthened by adding gussets in the corners that are glued and nailed with 3/4" brads.