Does it matter in which order I turn on the devices?
1. App on phone or tablet
2. Controller
3. Mavic

It won't blow up if you get it wrong but correct way is,
Starting Remote Control, app, aircraft.

Finishing , Aircraft Remote Control , App

It is also important to know if when you finish i.e. Land if you don't turn off your aircraft i.e. Motors are still running and you switch of remote control then your aircraft could go into RTH mode, so finishing is important once you get into a routine it becomes the norm, a flight list always helps.

Here is a link to my checklist, it's for commercial but most applies to hobby flying.

https://forum.dji.com/forum.php? ... &fromuid=260008