Been there, done that, got the big certificate from Office of Patents and Trademarks.
Start to finish, with no glitches, will take at least 6-8 months. (The publication in Trademark Gazette is a 30 day period but takes more like 60 days.)
First do a search on the USPTO website to look for any similar marks, as it may preclude you from going any further. (The examining attorney will kick it back if he/she deems there is a likelihood of confusion with a registered mark.)
Then it’s simply a matter of following instructions. A good trademark attorney will save you time but cost you $$.