First of all, there is a large school of opinion that you don't have to back framed canvases. Enough said. I am in that school.
OK, you outline the backside perimeter of the frame with adhesive transfer tape, using one of those enormously irritating Scotch Adhesive Transfer Tape Dispensers, an invention of Devil to keep framers from ever becoming happy people. Bring the edge of the tape to about 1/8" of the edge, all the way around. Lay the frame down on a piece of Tyvek, or Kraft paper, or whatever, and touch one of the sides. Now slightly tension the opposite side of the Tyvek, and lower the rest of the frame. Kind of like stretching canvas, only different. You'll develop your own little tricks for doing this step, it's a personal sort of thing.
Now, here's the fun part. Grasp the #11 blade of an Xacto knife between your thumb and the next finger. Now place you middle finger against the side of the frame, with the point of the blade about 1/8" in from the edge on the back side. Kinda smoosh the middle finger against the blade holding fingers, so all three fingers of a single unit, and use that configuration to guide the blade along the cut. Zip, zip, zip, and zip. You're done! Have the Bactine and a bandaid handy.
Alternatively, you can slice and dice with one of these, look about 40% of the way down the page...http://www.framingsupplies.com/ToolsTapesGlues/ToolsTapesGlues2.htm
Tyvek is the right choice, if you use paper your clients will put their fingers through within 2 seconds of being handed the frame. Of course, strictly speaking, nothing at all is the right choice. But that's just me.
Edit...oh wait, you probably meant unframed gallery wraps. Just don't do it. If there is any moral obligation to back canvases, it is only when they are placed in picture framing frame, and then only arguably so. For one thing, if you are using stretcher bars with adjustable corners, the Tyvek will turn a simple tightening operation sometime in the future into a total PITA.