I learned this some time ago and just used it again yesterday. If you are finishing off the back of a piece you've framed and you're adding a dust cover (you know, the brown kraft paper on the back), here's a few steps to get a good 'fit'.
Lay the frame on the kraft paper (or inside of a paper grocery bag) and draw around it with a pen or pencil.
Cut just inside the drawing (or it will be too big).
Using white glue, stick down the paper to the back of your piece.
Chances are, it will not be that tight or smooth. Use an old credit card or bone folder to get good firm contact with the frame. Now here comes the "magic" part. Take a spray bottle and fill it with tap water. Spray the paper until it's nicely damp and then leave it to dry. As it dries, the paper will shrink and pull it tight and snug. Here's one I did yesterday.
Then add your sawtooth hanger or hooks and eyes. With hooks and eyes, place them 1/4 to 1/3 of the way down the frame for it to hang nicely. Add a few of the rubbery bumper dots at the bottom to counteract the projection of the hanging hardware and you'll have a picture that will look almost professionally framed and will hang well on the wall.
Now, if like the one above and the edges aren't nice and clean, apply washi tape, ribbon or other finishing material to cover the edges.
I'd love to show the front of this piece as it is ones of the best paintings I've done, but it's for a Christmas present and she just may see this!