Portal 2

Chapter 2, Chamber 6.

After pressing the button for the first time, a turret should end up on the platform between the two main surfaces. Take this turret from the small platform to the beginning of a level (through a portal) and put it on the pusher plate at the start. Achievement unlocked!

Like stated in the video, there are other chances within the game to get this achievement, like chapter 3, chamber 15.

Note: If the turret is not in between the platforms nor at the end of the level, it may have fallen into the goo. Just restart the level, and keep track of it coming out of the disposal.

I have been going back to do the achievements in Challenge Mode.

Chapter 2, Test Chamber 6 in Challenge Mode is found in The Cold Boot: Trust Fling chapter.

If you missed your first opportunity, detailed in the solution above mine, you'll have another (possibly even easier) chance coming up.

In Chapter 3, Test Chamber 15, you will come across a room with a light bridge blocking the view of several turrets. Once you've dispatched a turret (the easiest being the lone turret that blocks your way into the larger room), simply carry it into the next room and place it on one of the faith plates. There are two in this room, one on either side of the back wall.

It's that easy! Just don't forget to continue the light bridge into the next room via portal or you'll get shot to bits the moment you set foot into it.

Chapter 2: The Cold Boot. Test Chamber 6.

When you make your way across to the button, press the button and GLaDOS will dump some garbage instead of a cube. The Garbage will come towards you (as long as you keep your two portals on the angled platforms). Stand in front of the garbage so it hits you. Shoot a portal to where you started and grab the Turret. Go over there and drop the turret on the Aerial Faith Plate. The Achievement will unlock.

I managed this in Chapter 9, yes nine!

After the defective turrets, and then a section with about 4 turrets in a room you can't reach, you will come to a large room with about a dozen turrets.

You know you are close to the large room when you come to a part where a pipe smashes the catwalk, leaving some blue gel, which you must use to make a somewhat tricky jump across the broken catwalk. Now would be a good time to make your own save.

I'm not going to spoil this, but once you figure out how to deal with the turrets (you've used a similar method already), the idea is to leave at least one turret behind, but on its side (this will make more sense when you figure out how to deal with the turrets). You can then portal over to the girder at the lower right of the large white wall. You'll need to make a jump around the white grid to the lip of the platform, grab the turret, and jump back to the girder. Now just place the little guy on the Faith Plate.

I found the catwalk jump a lot trickier than the girder jump. And even if you miss the girder, there's not that much to redo.

PS. This is probably not as easy as the other two spots but at least you don' have to replay an area. If you do want to replay an area, keep in mind that you can use the "Developer Commentary" option (on the Single Player menu after you beat the single player game) to reload specific chapters, but you won't be able to save in this mode. I'm also not sure what happens if you die... I used this method for Final Transmission and it worked out fine.