I attach all my ladder stands to the tree with a piece of 4" long by 1/4" flat bar stock with a slight bend in the middle. I then drill a 1/2" hole in each end. I use 6" lag bolts to attach the flat strap to the tree just behind the seat across a piece of the flat bar used to make the stand.
I make sure the ladder legs are down in the dirt solid before strapping it in place. I then take 2 3' foot pieces if 1/2" rebar bent like a candycane and criss-cross them across the lowest step. Cant pull the ladder up or the stand down.