A few years ago I swapped a 105 2x10sp setup for a SRAM Force 1x11 on an old CX bike. For that project, I needed:
- 11sp shifters/brake levers (you can use 2x11 shifters, but the 1x versions remove the shifting components from the left side, so I guess they save a little weight? They're cheaper)
- 11sp 1x rear derailleur
- 11sp cassette
- 11sp chain
- 110bcd narrow/wide chain ring
- single chain ring bolts (they're shorter than the 2x bolts)
- new brake and shifter cables/housing
- New bar tape (you could probably reuse old, but this is a good time to replace)
I didn't replace my crank set (FSA) or bottom bracket (BB30) for this. I'm not sure why it would be needed. I just removed the two chain rings that came on the bike, bolted on the new chain ring (I used a 42T from Wolftooth) directly onto my crank spider. I don't think I even needed to pull the crank off. All said, it was a pretty easy project and I'm super happy with the end result. Maybe took me 2 or 3 evenings and a half dozen beers to complete.
EDIT: The only snag I ran into was learning that chain ring bolts come in different lengths. The ones I bought were too long (for a double chain ring) and so I had to buy shorter ones designed for a single chain ring.