Elex

Teleporters show as purple dots on the mini-map, and they are quite easy to miss. You must walk right up to the teleporter and you will get a message saying it has been activated. You only need to have found 1 to earn the achievement, however the first 2 are near each other.

The earliest teleporter is outside of the Ruins, when you first meet Duras he stops outside ruins and offers you a chance to explore. Look for the purple dot and activate it.

The second teleporter is outside the town of Goliet, just near the bridge

Now to use teleporters, press then to map, scroll to a teleporter and press to teleport.

The other guide is great, and will help you grind the achievement out. But in case you're wondering where they all are; check this link below out.

http://game-maps.com/ELEX/ELEX-Teleporters-Map.asp

You'll likely get this achievement naturally while playing the game as intended, but if you want to grind it out fast, just unlock a teleporter, and while standing on it, open your map. From there, you can just press to teleport. Do this 20 times. This won't take more than a few minutes, as fast-travelling to where you are currently standing elicits only a short loading screen.