1)  Fold the sheet in half, then unfold. It's suggested to face
the convex side of the crease up as shown, as this will preserve
its direction in the finished airplane, but facing it down is ok.  
2)  The angle of the main diagonal folds (two symmetrical
folds, the first of which is shown above) is essential to
the perfect Sharky. It has to be very close to
30 degrees, with very little latitude for error.
The good news is that if your angle isn't perfect you can
still usually turn your plane into an excellent Sharky variation.
The correct angle can be generated using the linear
dimensions shown on the next page. The translation of the
diagonal folds along the length of the page may be varied slightly.
Images, Text, and 3-D Forms Depicted 2001 PaperAirplanes.net