The toilet can be found in the "Tranquility Node 3" section of the Space Station. Simply interact with the toilet in the cubicle to unlock the Achievement. You can also fast travel to different locations using the "Map" tab on your tablet.
There's also a collectible book in the cubicle as well that you can interact with to count towards the Book Worm Achievement.
Here's my full game achievement walkthrough showing you how to unlock every achievement as quickly as possible in around 25-30 minutes.
There are timestamps below as well to each achievement.
Timecodes:
Intro 0:00
Play Time 1:00
Book #1 1:15
Book #2 1:35
Handyman 1:45
Vomit Comet 2:15
Book #3 3:10
Pit Stop 3:27
Book #4 3:39
Book #5 3:50
Book #6 Book Worm 4:00
Straight Shooter 4:15
Home Sweet Home 3:40
Space Face 6:10
Shutter Bug 6:20
Cut The Feed 6:47
Earth Gazer 7:31
To See Or Not To See 8:35
No Pain No Gain 9:55
In Space No One Can Hear You Scream 10:15
Cargo Catcher 11:15
An Apple A Day 12:30
Station Explorer 13:25
Principal Investigator 14:40
Frequent Flyer 15:10