Choose a mat color that complements your painting. This can be done by selecting a secondary color in the painting and matching it. Cut the mat board to the size of your frame.
Turn the watercolor upside down on a clean surface. Measure 1/3 of the way in on either side of the paper, and 1/4-inch down from the top. This is the location for your t-hinges.
Prepare the archival tape according to the directions. Place one 3-inch piece sticky side up parallel to the top at the left location, and one at the right. Place another 3-inch piece of tape sticky side down in a capital "T" shape perpendicular to the first piece of tape. Apply pressure to ensure the tape is secure.
Measure and mark guidelines just inside the corners of the painting in the center of the matboard. Turn the painting over and place it according to the guidelines. With clean or gloved hands, apply pressure to the location of the tape.