Using 2x4s or other scraps of wood helps keep the shelter off of the ground so snow and rain can’t get in the opening.
On the end opposite the entry hole, screw the wood on the 2” side so it’s taller.
On the end with the entry hole, screw the wood on the 4” side so it’s lower to the ground. This will enable any water to drain down and out of the holes drilled in Step 3. It will also help keep snow from accumulating on the roof
Opt for screws that do not require pre-drilling and also use a washer to ensure it stays attached.