First there is the tree. It gets torn up into little bits and re-assembled into a sheet of plywood. The sheet of plywood is then torn up into little bits and re-assembled into, in this case, my stretchers. The logical next step would be for my paintings to be torn up into little bits and made into something else, which would also in time be torn up, and so on...
But now on to the practical aspects of building stretchers.
I always do my cutting in the hallway outside the studio to avoid getting sawdust all over everything. This is particularly important if there are any wet paintings around.
The strips of wood I prepared a few days ago, and the trusty power saw.
Make 45 degree cuts with the higher bevelled edge outside.
Cut gussets for the corners. 1/8" thick doorskin works well - 8" for larger stretchers, 6" for smaller.
Wood and gussets ready to go.
Use a carpenter's square, wood glue and 2" nails.
7-8 nails per corner works well - not too many, but enough to hold it together. The 2" nails are long enough for everything.
Measure from corner to corner. If it measures the same both ways, the stretcher is "square". (Meaning not a lozenge shape - this works for rectangular stretchers too :)
If it is not square, it can be manipulted a bit at this point to make it square. 1/16" off is nothing to worry about; 1/4" off will need adjustment.
Glue and staple or nail the gussets. This will set the corners and help prevent the stretcher from going off square when it is moved around.
Final check: make sure the finished stretcher is not twisted. This is very important for the painting to hang flush to the wall - have you ever seen a painting which has one corner lifting off the wall? Right.
I find the easiest way to check this is to free-stand it on the floor and eye it from above. The top and bottom bars should be parallel.
Free standing is important because if you are holding it, it may bend a little. If it is laying on a table, the surface of the table may not be flat.
If the stretcher is twisted, you can twist it the other way to get it straight. Usually twisting until you hear a slight crack will do the trick, but you will need to eye it again to be sure.
A magical process of deconstruction and reconstruction: 2 sheets of 3/4" plywood become 12 stretcher frames.
Final note: I generally cross-brace the rectangluar stretchers one at a time just before I am ready to use them. I don't know why I don't do this along with the rest of the process, but there it is.