First, fold the paper [by the way, it's much easier if you use tissue paper or some other thin paper!] diagonally to make a square (above) and cut off the too-long end:
Then fold it about one-third of the way in (from the center of the bottom side of the triangle) like so.
Fold the other side in so the paper is now folded in thirds. If you miscalculated your first fold, you can adjust it here so that each third is equal.
Now fold in half (lengthwise). Like this:
When you open it up you can see that part of the paper is shorter. Re-fold and cut the whole snowflake off across this line so that all of the sides are the same length.
Like this:
Lovely! Now you are ready to cut patterns in it!