The best way to use white glue, I use plain old Elmer's, is to lay it on with a short nap paint touch up roller. Get a piece of corrugated cardboard about 8"X 20", pour a small amount of white glue on it, load your roller and lay an even coat on the back of the frame.
If your using Kraft paper, mist it with water first, so the fibers relax. Lay this damp Kraft paper onto the glued up frame, pull it tight and trim it. Have a wet piece of terry cloth handy to clean up excess glue.
Wash your roller out or toss it into a can of water, then wash it at the end of the day. You have to spin it off before you can use it again.
Do not throw away the corrugated cardboard. Let the glue dry and just keep using it over and over, more you use it, the better it works.
be aware that some mouldings will require pre gluing. Old barn wood and mouldings that have a lacquered back or a finish. Lay a thin, even coat of glue, let it dry completely, then apply a new coat of glue and your dust cover. A lot of LJs mouldings require pre gluing. It is no big deal, a thin coat of white glue dries in a few minutes.
John