First, punch 3 small holes near the edge of the lamina using a nail. ( make sure it's big enough so that it can rotate freely on the nail) Place the nail horizontally and make sure that it's stable.
Hang the lamina on the nail at the location of the holes you made and tie a string attach to a pendulum ball on the nail. Wait awhile until the string is stable and make a point behind the string( the point should be as far as possible from hole that you hang the lamina from). Draw a line from the hole to the point you made.
Repeat this procedure with the other holes and the point where the 3 lines intercept each other would be the center of gravity of your lamina
Hang the lamina on the nail at the location of the holes you made and tie a string attach to a pendulum ball on the nail. Wait awhile until the string is stable and make a point behind the string( the point should be as far as possible from hole that you hang the lamina from). Draw a line from the hole to the point you made.
Repeat this procedure with the other holes and the point where the 3 lines intercept each other would be the center of gravity of your lamina