Measure the width of the vanity cabinet across its back. Add the desired amount of overhang measurement on each side if applicable. If the vanity is between two sidewalls, measure the distance between the two walls. Transfer the measurement to a piece of template material.
Cut the piece of template material using a standard razor knife. Place the template material across the back of the vanity, and position it so that it is tight against the back wall and side overhangs are correct.
Staple the template material to the top edge of the vanity base using a standard staple gun. The staples hold the material in place until the template is finished. Placing one staple on each end is sufficient.
Measure the front edge of the vanity cabinet, and add the measurement of applicable overhangs on each end and the front. Standard vanity counter tops overhang the front edge of the cabinet about 1 ½ inches, but the amount can vary depending on the application. Transfer the measurement to a piece of template material, and cut the template material with the razor knife. Position the front piece of that template material on the vanity cabinet. Adjust the template material so that the front and side overhangs are correct. Staple the template material into place.
Measure the two sides of the vanity cabinet, and transfer the measurements to pieces of template material. Cut the pieces of template material to length using the razor knife.
Glue the template pieces together using a hot-glue gun. Apply the hot glue on top of the left corner of the back and front pieces of templates. Set one of the side template pieces in place, and verify that it is tight to all sidewalls, if applicable, or flush with the ends of the front and back template pieces. Repeat this step for the other side of the vanity's template.
Label the template with a permanent marker to provide locations of finished edges, walls and the center of the sink opening. This information will let the fabricator know which sides of the counter top require a custom edge and where the center of the sink is located.
Remove the template from the vanity cabinet by removing the staples with a staple remover or small pry bar.