When you'll be having control over the protagonist, go right. Soon, you'll reach a box, in which lies a sack. Go pass this obstacle and proceed further, until you reach the lantern. Light it with your lighter and approach the hatch (open it using the "grab" button"). Then crouch and get into the tunnel.
Keep going right until you reach another location. Take the stairs and go up until you reach a gap underneath the door. Crawl through and you will reach a room. Jump onto the suitcase and the bed, and then jump again and grab the edge of the shaft in the wall.
Get to the other side and approach the chair - move it so you're able to leap to the door handle. Enter the next room, approach the fridge and open it. Climb to the top using the fridge shelves as a ladder. Go right.
When you reach the edge, a worm will drop right behind you. It's mortally dangerous, don't get close or it will kill you. Jump onto the other side, go pass the corridor with more filthy worms, jump onto the box and move the lever. Go through the open door and jump to the other side. After a while, you will arrive at a barricaded door and the passage in front of you will shut closed. Approach the plank and pull it out. Crouch and got through the passage you created - after a few more steps, you will fall down.