Unfold the napkin completely.
Because it already was folded like a square, you don't have to do that.
Fold all of the corners to the middle.
Rotate it so you have one corner facing you.
Again fold the corners to the middle.
Turn it around...
... and fold the corners to the middle.
Here comes in the glass. Lay the napkin - flipped upside down - on the glass.
Unfold the triangles like this.
Adjust and you're done!