Start with a square piece of paper.
Fold again, but by the other corners.
Unfold. You should have an X.
Fold in half again, but in the other direction. This time, do NOT unfold.
You should have a V, with the bottom point facing you.
End up with this. MAKE SURE THAT YOU HAVE TWO FLAPS ON EACH SIDE OF THE DIAMOND.
Squish it back into a diamond.
Unfold, but not all the way. Crease.
Tuck the smaller sail into the hull to hide that ugly crease.
Flip the boat over. Fold the bottom of the hull to the top of it.
Unfold, but not all the way.
YOU ARE DONE! Decorate desks, make oceans of sailboats, give them to friends... or not. The choice is yours now that you are an origami sailboat master! And don't forget to have FUN!