First, let me acknowledge the amazing Claudine Hellmuth--most of this process is taken from her wonderful book, Collage Discovery. I've tweaked it a bit to suit my style, but I owe it all to her... Second, I want to credit Joel with the pictures--he's home sick today and I pressed him into service...
Before any photos were taken, I had purchased strips of wood at Home Depot. The ones I use are plain strips of wood used for lattices. The cost is 73-93 cents a linear foot, which seems steep to me, but obviously it's much less expensive than having a canvas framed, so I just gulp and keep going. I buy it in the total length I need, to minimize waste, cutting the pieces to a total length for each canvas right there in the store. For an 8 x 10 canvas, I would need 2(8 + 10) + 3, or 39 inches of wood (the three inches I added was for waste cutting, and to allow me a bit of a cushion).
I bring the wood home and paint it with a small roller and semi-gloss black paint. It usually takes a couple of coats to cover--don't forget to paint the sides, too, since they'll show as the front of the frame!
In the first two pictures, you can see how I measure the wood, side by side with the canvas. I measure the top and the bottom first, carefully marking with a pencil, since an 8 x 10 canvas is not always perfectly square. Measuring directly like this also accomodates the thickness of the folded canvas cloth on the sides of the stretched canvas.
I cut the wood using a miter saw, but I've also used a simple miter box and hand saw and they work just fine.
I apply Elmer's glue and attach the wood to the canvas using small finishing nails. I didn't take a photo of this part (and it's totally optional), but I usually use my Dremel to drill pilot holes for the nails to prevent splitting the strip of wood.
After I've attached the top and bottom strips, I go back outside to measure the side pieces, once again holding the canvas standing up and marking the length. This will make sure that each side is exactly the right length (and that it includes the thickness of the top and bottom pieces you just attached!). I do the whole process over again for the two sides.
The final step is to paint with my black paint over the raw edges that are showing on the top and bottom of the frame. If necessary, I'll fill gaps with some spackle before painting, but my theory is that this is SUPPOSED to be a very primitive looking frame and generally I don't patch.
I hope this is a clear explanation of my process. If anyone has any questions, fire away and I'll do my best to answer them.