To stretch paper you need will need a board at least 2” (50mm) larger than your paper and some 2” (50mm) gummed paper tape ( the kind you wet to make it sticky)
Cut 4 lengths of tape to fix all sides of the paper with enough to overlap at the corners
Next you need to soak the paper. You can use a bath, sink or shower. Make sure the paper is thoroughly wet on both sides and leave it to soak for a minute or so, re-wetting if necessary. Be careful when handling the wet paper – use shower/taps on a low gentle setting – it is easy to damage the paper surface and sizing by rough handling.
Lay the paper centrally on the board and allow it to stand for a few minutes. Dab any excess water off (if there is water on the surface the tape may not stick properly) and stretch out the paper if it has buckled at all.
Starting with a long side and working round dampen the tapes and stick the paper down firmly. Check after you have finished that all the tape is still pushed down firmly then leave the board to dry horizontally. This stops all the water flowing down to one end and lifting the tape there. It will take around 3 to 5 hours to dry – don’t try to rush it by heating it as this can lead to the tape tearing or coming loose. Your paper will now be taut and flat – an ideal surface for painting