Assemble the four sides of the frame flat.
Cut the string to the perimeter size +? 10cm for the knot.
Apply wood glue to the corner surfaces in pairs.
Put them in contact and press a few moments.
Adjust the string around the frame, ending with a double knot.
It is useless to stretch the string to the maximum.
Ensure the tension of the string by placing plugs as close as possible to the angles.
Check the squareness and allow to dry.