The job is a PITA btw but if you want it doing check out JW AutoTrim on eBay. £40 per side if you send them to him. Highly recommended service (no connection to him btw).
This is a pretty good guide > https://www.pelicanparts.com/techarticles/BMW_Z4M/153-INTERIOR-Seat_Belt_Reel_Replacement/153-INTERIOR-Seat_Belt_Reel_Replacement.htm