First you need to find the spot on the outside of the grid that has been welded together. Do NOT try to bend it along that side or the weld will break and then it leaves a sharp edge. So when you find that side then you need to have that side pointed towards you when you bend it.
Now lay the grid down on the floor (carpeted flooring is best) and place your feet along the edge that has the weld (remember that side is facing you). Where your shoes end is pretty much where it will bend so keep this in mind and decide if you want it to have one bend along the middle to make an A frame or two to make a square tunnel.
Then grab the opposite side with both hands and pull it towards you. The weight of your body will be enough to allow it to bend easily.