Pretty easy achievement. EDIT: I've recorded a video showing how to both get to the Ancient Ruins and the Glass Desert. I will try to get the video into this solution.
Once you have one plort from all of these: Tabby Slime, Boom Slime, Honey Slime, Rock Slime, Phosphor Slime and Rad Slime, it's just a matter of getting them all through The Indigo Quarry and lastly to the entrance to the Ancient Ruins (you'll notice if you're at correct place, trust me). Once you're there, you'll have to shoot each different plort into its correct statue (each statue is of a different Slime). Once you've done that, each color above the big door, will lit up (as well as the one in the middle) and shortly after, the door will open.
Achievement unlocked. What are you waiting for? Go explore the Ancient Ruins!
NOTE: the teleporter that I use in the video, isn't necessary. It's just for getting there faster. The place the teleporter takes me to is the Indigo Quarry.