Spray down the back of the paper with clean water.
Flip and tape right side up using 1 1/2" glued paper-tape on a bigger piece of masonite board. Weigh sides of the taped paper down with books or video tapes ...anything flat with weight ... along the edges to ensure edges stay put. Don't get whatever you're weighing it down with ON the finished painting. Nust on the brown paper tape. By the way, you can buy that tape at art store, on Amazon or at the Post Office.
Next time, tape the paper down before painting. No need to pre-moisten the paper. Tape it down dry using moistened paper tape that is at least a couple inches on each end longer than sides of paper so it has a really good grip on the masonite.
Always shake the moistened tape off real quick AWAY from the sheet if paper before approaching it to tape down because you want to avoid getting the excess watery glue that was on the tape on your painting surface because ...it will dry clear and it is gonna repel your pigment when the pigment dries & you'll see 'mysterious' lightning shaped white marks or splotches where the dried cleat tape's glue splashed on your painting surface...so shake the excess water off the tape AWAY from your paper. I don't 'dip' the tape ... I swipe it really fast under hot running water outta the faucet in the kitchen sink, outstretch my arm AWAY from my masonite board that has my awaiting paper on it and give that tape a good shake...THEN, I tape it down on the edge, avoiding passing the tape over the paper...
Approach it from the SIDE. Do this on all 4 sides. Overlap at least 1/2 an inch onto paper edge when taping paper onto masonite board.
Do it right the 1st time so tape it straight as you can.
Tape the top, then 1 side, then the bottom, then the other side...not opposing sides 1st.
This ensures a tight stretch.
Only use the really WIDE papertape. 1 1/2" roll of papertape. Buy at post office if you can't find elsewhere.
When completely dry, then do your painting.
When finished, cut just onto where you see the paper under that tape...cut through tape into the paper with a box cutter on all edges & press hard enough. Careful you don't cut your painting image.
Carefully lift it off the board.
Where the tape remains, you can just leave that because the matte is going to cover it anyways when you or your client goes to frame it.