The first thing you need to do is to go to the options menu from the main menu.
On the right side you'll have "Console (~, ingame) OFF", click on OFF to change it to ON.
Now during a game session press the ~ key (top left of the keyboard, below Escape) and enter one of the following codes, followed by Enter:
Level skip - nextlevel
previous level - prevlevel
Set gravity - gravity [number]
Suicide - kill
Go through walls with [Cursor keys] - noclip
View game information - info
Toggle sound - sound, sounds, or autoquality
Hold [Space] to open a portal - space, aim, or detectshot
Set quality level - quality [0-3]
Set effects level - effects [0-2]
View frame rate and level number - showfps
Toggle God mode - god
Toggle Portal Proof mode - portalproof or antiportal
Toggle dancing - dance or /dance
Toggle crosshair - crosshair
toggle Playercam - dynamiccam or dynamicam
Stop all sounds currently running - stopallsounds or stopsounds
Toggle map box - showmapbox
Toggle display of Portal box - showportalbox
Toggle display of Portal hit box - showportalhitbox
Toggle display of Portal mask - showportalmask
Toggle step visibility - showsteps or showstep
Toggle shot visibility - showshot or showshots
Spawn Box - createbox
Remove Box - removebox
Spawn Cam - createcam
Remove Cam - removecam
Spawn Ball - createball
Remove Ball - removeBall
Remove Turret - removeturret
Restart indicated level - restartlevel [number] or restartmap [number]
Use gravity to give you an extra boost.
Shoot a portal in the roof. Shoot one under you. Now you should be continuously falling. While you are falling, shoot a portal on the bottom next to the tower. Your extra momentum while falling will give you an extra boost when you come up. Next tower.
Shoot a portal to the left side, as high as possible. Shoot another portal at the bottom right. Jump in. If you don't have enough speed to clear, aim for the bottom portal.
Second time should be fast enough.
This one is kind of hard and requires timing.
Stay on the high ledge. Shoot a portal into the middle (a bit to the right of middle), shoot a portal to wall above the entrance. Jump from high ledge into lower level portal. You should have enough speed to shoot across. Now you're in the middle ledge. Shoot a portal under you. You should reappear back on the high ledge. Shoot a portal into the middle of the lower level like you did the first time. Jump in from the high ledge. You should have enough speed to fly up from the middle portal into the energy section. So timing is crucial here, so jump in as it's about to go off. Once there, hop to the right and you're clear.
Stand as far right as you can. Shoot portal above red button.
Shoot second portal under the cube near you, landing the cube on the red button. Walk through the portal and stand on the other button. The door will open, now shoot another portal on the far right wall, and one on the wall to your immediate left.
Pick up the cube (F) and walk through portal. Place cube down (F) on the button to open the bottom door. You can't move through the blue line, but you can shoot through it so open a portal anyway beneath it. Walk through and exit.