Another reason to do alla prima work on a taped-down piece of linen is that I don't always get the composition exactly the way I want it. For example, if I'm painting a profile, I'll want to put the head on the canvas so that there is plenty of "looking space" in the direction where the sitter is looking. It's annoying to be two hours into a portrait head study and find out you should have moved the head an inch or so to the right. This kind of thing drives me crazy. I hate not getting the design right on the canvas! I often start the whole thing over if that happens.
So, let's say you have a painting on a piece of linen and you want to glue it to a panel. Measure out the dimensions on the linen and then cut around the piece - give yourself an inch or so for the final cut, don't cut the piece exactly to the size of the board and then try to glue it down so the edges match, it will drive you insane.
Piece of masonite (I think this might be Ampersand board) on the left, linen on the right, on top of a scrap board so I don't carve into my nice table.
I put matte acrylic medium on a sponge and I wipe it on both the board and the back of the linen. Then I put the jelly side of the board to the jelly side of the linen. I push down with my hands and then I flip the jelly sandwich over.
My skull is stolen from the novel Lord of the Flies, by William Golding. Not to be confused with the novel (and movie) The Lord of the Rings by J. R. Tolkien, which is another thing entirely. This is not my own skull, either, I have more of an overbite.
I flip the painting over and put tissue paper on top of the paint. Then I roll the painting with a rubber brayer, being careful around any impasto areas.
Then I flip the whole thing over again and put heavy books on top to weigh it all down overnight, so it's a little like salt-and-dill-cured gravlax, if you've ever made that. I always hope that some osmosis will take place and some art wisdom will seep down into my paintings. As you can see below, in this case I had a book about Sorolla and one by Vincent Desiderio. I recommend both of these books to read, not just for pressing.
Let your sandwich dry overnight and then you're ready for the final trim with a sharp x-acto blade. Be careful!
Sometimes I wrap these boards like tidy packages, with neat little hospital corners, but it's much more sticky and fiddly. If you have had the foresight to see if the board fits into your frame properly, you may need to add the tiny bit of fabric afforded by the fold. Framemakers are maddeningly non-uniform in their ideas of what constitutes a proper size frame for, say, a 20 x 16 painting, one of my favorite sizes. One of the bad things that can happen is that the whole belabored board and linen sandwich is too big for your too-tight frame, and like Cinderella's evil stepsisters on confronting that ridiculous glass Barbie shoe, you'll be forced to make a Solomonic judgment on what to do about it.