A friend of mine gave to me an used canvas of some jewelry or perfume advertise. Although I had better wished she introduced me to the model, I thanked her and decided to make a fast wood frame and hang it on the wall. It's a very simple instructable, but anyway it could give you some cue to reuse a nice advertising poster.
You need to take measures of the canvas, and assume a thickness for the wood profiles. If you don't find the right thickness, remember that the more thick is the profile, the shorter its length should be. This is because you have to turn down behind the frame the edges of the canvas.
If you buy the wood frames in a shop with cutting service, you could make the cut the right length, and avoid some work with hacksaw and angular guide.
Then you need a staple gun. Mine is very cheap because I rarely use it, but it's very handy if not essential for some projects.
Put all the four profiles at right position, and check they form a perfect right-angle rectangular. You can glue them together and keep them in position with some paper-tape.
Now fix them shooting one staples on the first side and two staples on the opposite side. In this way you'll be sure that the staples don't interfere one to each other. Then you can remove the tape.
Check the size is right for your canvas. Try to pull the edges to see how much inside the frame they reach, so you could pull them on the right position when you'll begin shooting the staples.
When you know how much pull and where the canvas edge should arrive, you can begin blocking a short side with one staple at time. Finish all first short side then begin with the opposite side. Here you have to pull a lot the canvas, then lock it with your finger and shoot the staple... not on your finger! Complete with this method the second short side.