You do not have to go to the story levels (hub worlds) and play the bonus level you edited!
To unlock this achievement:
1) Go into the Creator Hub
2) Choose Level Builder
3) Choose a empty slot in My Level Set 1 (Does not matter which slot, just choose an empty one) and select "Create"
4) Page all the way over to the left and choose any level you have unlocked in Story Mode (any mode but Creator) under the "Bonus Theme" tab
5) Pick any bonus level except the "Start with Empty Level" option
6) Select and enter the level
7) In the level, hit the Y button and choose any object, stud or item and place in somewhere in the level
8) Hit "Start" on your controller and select "Test Level"
9) Play until you gain 1,000,000 studs and the achievement will unlock.
Note: You can speed up the level testing (getting to one million studs) if you have unlocked the score X2, X4, etc. Extras in the Main Hub Worlds
You should see a message at the bottom of your screen when the achievement unlocks that says "You have validated this bonus level and it can now be used in build your own adventure mode."
Anything I left out? Feel free to message or comment.