"Welcome to Rapture" is the first level of the game. It consists of the very beginning of the game to the medical pavilion.
When it starts, swim to the tower and follow the lights to the bathysphere and pull the lever. When the bathysphere comes up and the cut-scene ends, grab the radio on the left side of the door. After the door opens leave and head to the area with the monitors and keep going until the camera sends security after the enemy (spider splicer). Now you're allowed to jump over the fallen pillar, grab the wrench on the ground when it starts to glow gold. Now smash the rubble and start up the steps, you want to stay to the left and walk slowly because the flaming couch is about to be sent down. Now there will be a thuggish splicer (bad guy with a melee weapon), take him out. Search around if you want, then head on up the steps and get the Electro Bolt plasmid and wait for/watch the triggered cut-scene.
Now go to the big door with "Welcome To Rapture" above it. There's a malfunctioning door control to it's left, shock it (LT) to get it to work. When the tail of the plain crashes through the tunnel you need to go into it and out the exit door (yes some people didn't know this...) and keep going. In the next room an airlock is active, walk around and pick some items up and a thuggish splicer will arrive. After you kill him another will come from the door with the red "airlock" sign above it. Keep going and kill the flaming thuggish splicer that came out of the exploded elevator and walk into the only open elevator (note: the door will lock when you enter this room if you weren't done scavenging). The elevator will automatically activate, when you come to the destined floor kill the thuggish splicer tending to the baby carriage. In the carriage is a revolver, pick it up and head on through.
It's optional to head down the stairs, but if you do kill the thuggish splicer by the door and then the leadhead (bad guy with a gun) splicer that comes out of it. If you scavenge the room she came out of there will be 2-3 splicers (usually 2 thuggish, 1 leadhead) standing in the water-logged area you can shock for a 1-hit kill. When you head back up a thuggish splicer will be waiting for you. Note: there's an audio diary on the table in the water-logged area you should pick up.
Whether or not you were to go downstairs, you will come to an area with split paths leading to restrooms. In the women's room (right) is an audio diary that should be picked up and an ambushing thuggish splicer. But the men's room leads to a theater-like room. Head across the lighting support and down the stairs, after the splicer get murdered and the lock on the gate glows break it by either whacking or shooting it.
Keep going and a leadhead splicer will try to attack you, dispatch of her and then a thuggish splicer will pull an identical attempt. Not much further you will see two thuggish splicers looting a big daddy corpse in the water, I recommend shocking them. Go to the open gate to be cut off, back away from it because a flamethrower turret will be stationed there. Now watch the area where water is flooding, a group of splicers will drop down here and should be shocked quickly, then after an explosion a second group that should be taken care of the same way.
Now just go the opposite way until Atlas says you're trapped. Wait for the door to open, enter and go to the Medical Pavilion. Achievement unlocked.
This guide is detailed for new players, people who aren't really accustomed to games, and just overall anyone who needs a little extra explanation/guidance. It's not suppose to be a further explanation of the achievement but an actual, very simple, guide/solution.
Any questions/comments feel free to message or leave a comment.