Chris is right of course, you can get the patterns from either NAFA or FHA, but IMO patterns aren't really necessary. The beaver will tell you how to stretch him, you just have to pay attention.
Staple the nose of your beaver at the top, then measure down to the half way point. At mid way, grasp each side of the pelt, pulling it out to the sides and staple in place. then staple the bottom (tail end) where it ends up. You now have a diamond shaped hide on the board.
Now go to the leg holes and pull them out (first the front legs, then the back), you now have eight points stapled and you should have a general oval shape (some are more round or oval than others, but always oval, but then all beaver are different).
At this point start nailing alternate sides (one left, one right, one top, one bottome again left to right)at the mid point between staples. When you get to the existing staples, replace them with nails and pull your staples.
At a point midway between the nose and eye holes drive a nail, and working in an arc, drive nails about 1 inch apart out to the nails just below the lip area, smooth it up, and trim away everthing above the nails (you are cutting away the lips and nose area, giving the beaver a rounded appearance at the top.
When you are done, you should have nails spaced about an inch or less apart all the way around the beaver). Then nail your leg holes closed. Once you have done that, pull the pelt away from the board to allow air to circulate freely between the board and skin, and you are done.
I like #6 Box nails for this, and it takes a lot of them, so get plenty.
Hope that helps.