This is very very easy!
These are not the stairs by the water it is the stairs under the "CatWalk".
1-First go up the stairs then jump onto a small ledge off the right side of the stairs (if your coming from the Seal Spawn, its on the right).
2-Then Prone right next to the small Wooden beam and crawl in it on the left side of the Wooden beam
3-When you are in, you can not be shot and you can shoot them, if any part of your body is sticking out then you can die. also the your team or the opposing team will nt be able to run past you only if they jump over you.