My husband is an artist and I just asked him. He said to go to a store that sells art supplies (Aaron Bros., Michaels, etc) and get some appropriately size stretcher bars. They go together with wooden wedges and he says it's easy. Once you get it together you can stretch the canvas around it and staple it to the back. He says to work from the middle out... like staple one side in the middle, then the other, then another side and the last one. Then continue around the edges.
If you have any more questions just ask.