Find a nice spot in the woods to lie down in or get into your fort. The best place to be is behind something to prevent getting hit. I prefer the fort because it is way up there and you are covered by the walls and ceiling. And there is still a window to shoot from.
If you are in a place where there are either no forest nor open space, use plywood to make walls skids or pallets work too.
To make the walls just simply lean the plywood against something and drill a hole in it big enough to see and shoot from.