Select the dimensions for the rectangular base according to your preference. Keep the size small for easier calculations.
Choose a height at the apex for the pyramid. Keep the height limited to no more than two or three times the longest length of the rectangle.
Decide where the apex should be for the pyramid. Choose any location in the base aside from the center. Express the location as units in from the left and units up from the bottom. For example, the apex for a rectangular triangle with a base that is 5 inches by 3 inches may be at 1 inch in and 2 inches up.
Assume that the height is one side of a right triangle and the distance from the edge of the rectangle is the adjacent side. Plug the distances into the Pythagorean theorem to solve for the hypotenuse. For example, if the height to the apex is 6 inches and the left face of the triangle is 2 inches from the apex, then 6 squared plus 2 squared equals 40. Find the square root to provide the length from base to apex of the triangle on the face. For example, the square root of 40 is 6.32.
Repeat the above step with each of the remaining three sides.
Draw the rectangular base on a piece of paper using the dimensions you selected.
Mark the relative position of the apex on each edge of the rectangle. For example, if the apex is 1 inch in from the left and 2 inches up from the bottom, mark the left edge of the rectangle at 2 inches from the bottom and the bottom edge at 1 inch from the left.
Use the length from base to apex for each face determined above. Draw a line at a right angle to the mark on each edge of the rectangle. For example, draw a line that is 6.32 units long starting at the mark on the left edge of the rectangle.
Connect the end of each line to the corners of its corresponding side of the rectangle using a straight-edge.
Cut out the design you drew. Fold the triangles up. Secure the edges of the triangles together with tape.