This one is really easy if you know horizontal clipping or PB&J. On the left wall of terrorist spawn (small map) theres a table infront of the bookcase. Horizontal clip through the table and it should pop you up on top of the table. Next, proxy slide through the wall while your on the table. Once you fall through stay in crouch and it'll keep you in. It wont pop you out because the tables in the way. You can see the tunnel outside the door of the spawn and the left tunnel coming into your spawn.