Roll the clay out with the rolling pin to desired thickness. Don't make it too thin or your triangle will fall apart when you try to use it in a project.
Place your ruler in a straight line and cut the clay to form the bottom of the triangle.
Place the ruler on the left corner of the bottom of the triangle and lay it diagonal to the right to form a 45 degree angle. Cut the clay to form the left side of the triangle.
Move your ruler to the bottom right of your triangle and place it diagonally again, connecting the bottom right corner with the tip formed by the last cut. Cut the clay one last time to form the final side of the triangle.
Remove the clay surrounding the triangle.