Watercolor paper expands when it is wet, creating an uneven surface to paint on. For control and a flat surface, artists soak and “stretch” their paper.  The paper is soaked in a tub of water for about 10 minutes then laid out on a hard surface, such as a plywood board and taped or stapled down. As the paper dries it contracts and becomes tight like a drum. Then the artist can paint on the paper with no buckling or dips. It becomes and ideal surface for watercolor.
Materials for stretching:
Use 3/8†plywood boards. Sand and seal board before using.  Paper Size                                                          Board Size                                                                     (Includes 2 1/2†border on all sides) Full Sheet – 22†x 30â€Â                                       27†x 35†Half Sheet – 15†x 22†                                        20†x 27†Quarter Sheet – 11†x 15†                                16†x 20†ÂOr, use thick (3/4â€) gator board. Gator Board is light-weight, can be stapled into and wet paper can be taped on.
140 lb watercolor paper (I recommend Arches) cut or torn to size
Paper towels
Squirt bottle of water
Kraft tape (a brown tape, 3 inches wide, that comes on rolls)
Or: Lukas Wet Adhesive Tape 40mm (1 5/8 in. wide) (Jerry’s Artarama) I haven’t used this yet, but one of my students says it’s great. It’s a white tape and can be used on wet paper to adhere to gator board.Â
Ruler
Pencil
Scissors
Staple gun
Instructions:
1)     Fold and tear (or cut) paper to size 2)     Cut tape to fit four sides of board 3)     Measure and draw one inch boarder on all four sides of paper 4)     Soak paper in tub for 10 minutes (set timer) 5)     Lay down paper towel on board 6)     After 10 minutes, let extra water drip from paper 7)     Lay paper on top of paper towel on the board 8)     Spray shiny side of Kraft Tape evenly with water. Hold up until tape straightens out. 9)     Lay tape on the one-inch boarder so that the tape overlaps the paper by one inch. 10)   For large paper you may also staple edges of paper to board to add extra strength. 11)  Let paper completely dry till it is as tight as a drum.         I usually let my paper dry overnight. 11) Paper is now ready for painting. 12) Do not remove from the board until you are completely finished with the painting. 13) To remove the paper, cut along the tape with a utility knife. 14) You don’t need to remove all of the tape from the board.  Alternative method: Stretcher Bar Stretching of Watercolor Paper 1) Buy 4 stretcher bars for stretching canvas… any sizes. (2 long, 2 short, or all the same length) 2) You will need to plan for a 1 – 1 ½†border. 3) Soak paper for 10 minutes 4) Staple (with staple gun) The middle of each side Then the corners (folding corners) Then filling in about every 1 – 2 inches, or so 5) Let dry. 6) This method will dry faster than the board stretching.Â