Draw a quadrangle on a piece of paper. Use a ruler to ensure that the sides are straight. The corners do not need to be right angles. Cut out the quadrangle with the pair of scissors.
Use the ruler to draw a diagonal line joining any two opposite corners of the quadrangle.
Fold the quadrangle along the diagonal line. A triangle is formed on each side of the fold line. Cut along the fold line to produce two triangles. Note that the triangles may not be identical. The sizes and shapes of the triangles will depend on the angles and side lengths of the quad.