Most of my 'backings' are colored papers I get from the 'Scrapbooking' section at Michael's. I have an assortment of various colors and just select what I think looks god on a particular project.
After the piece is cut and all finish is applied to the face I hit the back side with a simple coat of spray lacquer. Then A quick coat of 3M "77" spray adhesive. This adhesive was originally developed to install 'headliner' in autos!
Now I take the pre-cut backing sheet and carefully position it on the back side, press firmly and then use a rubber roller called a 'brayer' to roll it down good and tight. Once this is done that paper will adhere virtually forever!
If the piece needs no frame I attach a sawtooth hanger or if it is to be framed I put it in a frame, add a piece of 'chipboard' like the back of a paper tablet and then secure it in the frame with glazier's points.
~~~GrayBeard~~~