How do you get this simple achievement? Read this guide if you're stuck.
Honey, I Shrunk Myself! Achievement Guide
The first thing to notice is that in every space where there is a God Cube (a.k.a. "Mandala Rooms" since after placing the God Cube you see a mandala floating in the sky in the same space) there is also a tree that looks like this:
I recommend you find all 7 of them before moving on — they are not as useless as you might think. Inside you'll find one of those infinitely repeating hallways with a map that looks like this:
The other thing to take note of is your reticle. Apart from changing colors to indicate that you can change gravity and in which direction, any time you can interact with an object your reticle will have a pair of hexagons like this (which become filled in whenever you're holding an object):
If you see any darkness obscuring any part of the map, you'll only see one hexagon on your reticle, which means that the object is interactive, but nothing can be done yet. To get this achievement, you'll need to clear the darkness, which means...
...you need to plant the cubes to grow all 7 worlds!
Only then can you interact with the map to get this achievement. And fast travel too!