I assume you mean a SL Creo Comp Carbon since the SL 2 Comp has SRAM eTap and your are trying to go 12spd. Bettershifting.com is an excellent resource for all things Di2. He has an article
Building a 12-Speed Di2 Road/Gravel Bike - components and wiring that gives a great overview. It depends on skill level as to easy or difficult and you have to open/bleed the brake system to replace the shifter levers. On my Creo Expert, the Di2 battery was in the top tube behind the TCU. At this time, SRAM eTap is a much easier to install conversion. The SRAM components mix and match between MTB and Road. A shop may be more expensive but they should readily know which parts are required. I would expect it's somewhere about 1.5k USD to convert depending on where you source the parts. Unfortunately the best prices are in Europe but they can't ship to the US. A SRAM eTap mullet build is closer to 1k. Shop labor is extra.