Tetrahedron (modular)

1. Start with a square of paper. First make creases that divide the square into quarters.

2. Next fold two opposite sides to the middle line, and make a horizontal crease across the middle:

3. Fold top and bottom edges to the middle line, and open out:

4. Fold the point A to touch the line C, folding from B. Then open out:

5. Do the same for each of the other corners, so you end up with creases as shown:

6. Then open the whole thing out, so you have creases as below:

7. Fold over the top and bottom edges along a line which passes through the points where the creases near the top and bottom of the paper cross:

8. Next fold along the dotted lines shown:

9. Fold over along the line joining the points A and B, and then along the line joining the points C and D. Next make the creases along the lines from R to S, S to T and T to U:

You also need to be able to fold the mirror image version:

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Putting two units together:
Units fit togther by putting the point of one unit into the pocket of another, as in this example: