The focus mechanism has two guides in the helical that keep the inner helix from turning So that's from the back.
You're going to likely take it apart from the front and back. The infinity stop will be at the front and prevent it from unscrewing. And the front.

Assembly is going to be more complicated. You're going to need to mark the helicals with a scribe mark to get the alignment right the first time.
It's not critical but it can be REALLY frustrating if you don't. Keep in mind there are five or six threads on each tube and only one will allow the guides, aperture mechanism and infinity focus to line up. You may need to adjust infinity when you're done anyway.
*It's really fun when you don't mark it =\ but is good for teaching patience, grasshopper.