I purchased a SpinnTwinn tarp last year and after getting some residual rain splatter during a very heavy down pour, I decided to add beaks (Ray Jardine style). I'm not sure if this is needed by most, however, and I felt it was only necessary in my circumstance because I'm fairly tall (6'6") and when the tarp is pitched in the GG storm position (one end up and one end down) it takes away a bit of its length. So for me, in this position, my head was near the opening and the rain splatter kept me awake all night.
Adding beaks to this tarp is actually not that easy because it has all kinds of funny angles. I decided to work principally on one end since it would be easier. I contacted GG and got 40” of Spinn fabric and sewed it on with a similar angle. I then added a nylon strap on the end and put in a grommet. On this side, I kept the previous nylon strap from GG in place as it now serves as a place to support my pole when I’m using the beak. My new strap, when pulled in this position, then forms the beak. On the opposite end, I sewed another nylon strap about the same distance as the other end inwards within the tarp, so again, I would have a place to put my pole. I then had two functional beaks if I choose to use them since the ends already came to a point. I also needed to add additional tie-downs on the sides of the newly added fabric.
Personally, I like the tarp so much more now that I added the 40”, but it is pretty big and it is important to stake it down properly. After seam sealing everything, I’ve had a lot of luck with it in the conditions which I’ve tried it. Even though it isn't waterproof, I haven't had a problem with it in even the heaviest of rain. Rain does come through, but it doesn't dribble down. It just disburses.
Good luck.