Once you know how, this is a simple project - turning a flat rectangular piece of paper into a full tetrahedral solid.
Step 1: Thirty-Six by Five
That's the secret - you need a rectangular piece of paper or light card with sides in the ration of 36:5.
It can be 36cm x 5cm, 9" x 1.25", whichever you like. Obviously, the larger the piece of paper, the larger the model you can make. You can, if you want, use two pieces that are in the ratio of 18:5, then tape them end-to-end to make a larger piece*.
If you have some sort of graph paper, your job will be made a lot easier, because all you need to do is count squares.
* The next steps will assume you are using a single, long piece of paper, but if you've got the brains to check this footnote, you're able to adapt the instructions to using two separate pieces.