***************SPOILER ALERT****************
This achievement cannot be missed, as it's part of the story.
When you get to chapter 6, you will eventually come across GlaDOS in her potato form. She will ask you to carry her so you can both stop Weathley. Once you pick her up, a switch under her will activate a bridge and the achievement will pop.
P.S. this part is when you get to the "second" Aperture Science Lab, the one where you use the propulsion gel.
EDIT:
***Spoilers***
Actually there is a chance of accidentally skipping this. It happened to me yesterday -- there is a way for you to get to the elevator without needing to activate the bridge (and find GlaDOS). I noticed something was wrong when I started the next chapter and I suddenly had her in my possession.
Here is a video showing how you can skip this part (and not get the achievement):
Thanks to DarthMike431 for clearing this up, I wasn't aware of this.
P.S.: if you're going to rate down, please specify so I can edit the solution