This is a top view of a Y-board in a Y tree.  Note the 4 nails toe nailed into the tree.  Oh the tree doen't have to be a perfect Y.  A Y-board can fit on a single limb also if it is big enough to hold you and the stand.  Just nail it to the limb, and toe nail the Y end of the Y-board to the trunk of the tree.  I have one of these in a huge sumac tree.  If your in a state that doesn't allow nails in a tree or the landowner doesn't, then take two U-bolds, place one on each side of the V, and use rope to secure the V-Board to the tree.  Do the U-bolts on each end of the Y-board.  You can also jamb the Y-board in the Y-tree with a 2 lb mall.  You can also use a step on top of the Y-board to keep it in the tree, basically any fasner that keeps the Y-board in place.   Nail and screws are the best.  Rope thru the Y-board (drilling a hole say 1") on each side of the Y-, both ends, and threading rope thru and tying that to the tree.  Any way to make it safe and stable.  Do NOT get inot a ricky Y-board just loosley placed in the Y-tree.  It must be rock solid in the tree. It you can step on it and it doesn't wiggle or give your good to go, if there is ANY movement, you must solidify the stand via some method.  After 1 year, the tree grows around the Y-board so it becomes rock solid without nails or anything, but year 1 your going to have to secure it to the tree some way.  Chap Gleason