Choose your matboard color based on the colors in the artwork. For a soft, unified effect select matboard that closely matches a dominant color in the painting. To achieve a more edgy contrast, pick an important but less prominent color that occurs at least three times in the artwork.
Measure your artwork. For wide borders, add six inches to the height and width when determining matboard dimensions. Final borders will be three inches on each side. For narrow borders, add two inches in each direction. Final borders will be one inch on each side. Another option is to base matboard dimensions on standard frame sizes. Choose a frame slightly larger than your artwork and measure the matboard to fit the frame.
Use your border measurements to mark the matboard. Cut it with an exacto knife or mat cutter. Sometimes a representative from the art supply or framing store may be willing to cut matboard for you if you know the correct measurements before purchasing.
Place a ruler on the edge of the matboard and measure the width of the border. Make a faint mark with a pencil. Do this in two places along each edge.
Position the artwork within the borders. When it is positioned to your satisfaction, roll bits of linen or acid-free tape so that the sticky side is out. Tape the artwork to the matboard at each corner. If the artwork is large, add a piece of tape near the middle of each edge of the watercolor paper for extra hold.