While it is not totally necessary, stretching makes the surface of the paper lie flat and accept the paint without buckling.
For larger works, this is a big help, especially for heavy washes and large color applications. Methods and materials vary, but there is surely a method that appeals to you and the way you approach your creative work.
You will need a 1/2-inch sheet of plywood. This makes a heavy board. If this is a problem, try using stretcher strips that are tongue and grooved.
I make up sets for half and full sheets in size. The most widely used stretching method is the taped edge technique. You will need a smooth, clean board. This provides the type of surface against which the paper can lie until it is dry.
Begin by thoroughly wetting your paper. All papers but the very heaviest benefit from soaking and stretching. The best place to soak the sheet is in a clean bathtub. Allow the paper to lie submerged in cool water until you can gently curl up a corner with little resistance. If you over-soak a sheet, you run the risk of dislodging the sizing and changing the overall performance of the sheet. Five minutes should be enough.
Once thoroughly wet, lift the sheet by two corners and allow excess water to run off. Blot the surfaces (both front and back) with a clean sponge, removing additional excess moisture. Lay the sheet out on your mounting board and smooth with your hands.
Staple one corner of the paper to the board. Next, the second staple goes in the corner that is closest to the first staple. Now, staple with staples about 2 inches apart. You now have stapled one of the two short lengths of your paper. Now repeat the other short side.
But, before you drive in the staples, grab the corners and gently pull against the first set of staples, holding, drive in staples.
Now, you have the two longer edges to staple, grab in the middle and try to gently pull towards you. It will not move if any. Drive in staples on both sides 2 inches apart. Let dry overnight; paint on the sheet stapled down. Don’t laugh. I had one student remove it from the board. Then, back to square one.
Now, use plain old masking tape around the stapled edges. Happy Painting!
It is a chore to do this, but there will be no more valleys collecting the paint and no more buckles. The surface will be tight as a drum.