You need a square base folded by the middle and the diagonals (see how on the picture).
The difficult part is at the beginning, steps (1) and (4), (5) and (6), where the folded part has to be opened and folded inside. This is repeated three more times. When you are ready with the one side, turn the form and repeat the steps again.
Cut the lower part with scissors.
Make 5 symmetrical cuts at the two sides and start folding one after another, and your fir tree is ready.