Once you're side-kick has shown up, aim your gun at the wall behind him and place a portal here, now drop into the orange portal to enter the next part of the room.
Once you've picked Wheatley up, enter the newly opened passageway and follow the - very - linear path until you enter two double doors open and the screen fades into the next area. There's no real way to get lost in the next area as there's only one way to go (and no puzzles to solve).
Once you set Wheatley into the device, the lift you're both in will start raising. What follows is a cut-scene where you can't do anything, so sit back and relax until the achievement/trophy 'You Monster' pops up, signalling the start of the final part of this chapter.
Walk across the metal girders carefully and you'll soon come across the proper double-portal gun on the ground, this is where the real challenges start to take shape as you now have to lay both portals yourself.
After landing, look to your right and fire a portal off at any part of the wall past the metal barricade and then set the entrance portal right in front of you. As you make your way around the corner of the corridor, GLaDOS will open up a slanted wall for you.
Fire a portal up here and then an entrance portal on any nearby wall, step through and then drop off the platform you're on to the ground down to your left.
Hang a left as you exit the portal and go around the corner, where a bunch of debris will be removed from your path. Around the next right-hand corner are the steps leading to the lift, signalling the end of Chapter 1 and the start of Chapter 2...