Hi John. This question comes up frequently so you'll hear the best of what works from each of us. I always use wood glue if the backer is wood or some form of a wood product. If the project if framed I just spread a few drops of glue very thin to the project, just enough to hold it in place. No need for more than that. For larger, unframed projects I do the same thing only add a few extra drops in critical places. I uses weights or clamps to hold the backer in place until the glue dries. With practice you'll figure out what the right amount of glue is. I go very light with it. Never had an issue. I did this owl in 2013. It's about 3' tall and 1' wide, with picture matting material, glued with Titebond II. It's still hanging today.