M.L.
We started doing naps in the new room first with no decorations in the room. After she was comfortable napping, then we added night sleeping still without any decor. Just painted and with boring blinds. After about a month of that when it was going well, we added decor. She transitioned from each phase seemlessly and didn't seem overwhelmed by all the new at any point.
Also, the more you push it, the more likely she is to push back so let her do it at her pace. :) Good luck!