Good for you , you did not settle for the lazy mans textured walls. Use a nap for smooth walls before painting prime well with drywall primer I usually do 2 coats . Watch painting videos there are great tips .

Thanks Patricia! Any videos you suggest specifically?

keep trying to find a pro with experience it's not easy to paint smooth walls. it can take years to learn to paint smooth walls correctly. the chances of leaving lines and leaving the paint to heavy is highly likely. if you must paint them yourself use a shed resistant 3/8 nap roller a Purdy white dove is a good one. I wouldn't recommend spraying if you are a novice when it comes to spraying.

I can't speak to an Old Masters level of precision, but those of us who grew up where smooth walls were the norm weren't too paranoid about them -- when we wanted to paint a room, we just painted it! Roller for the bulk of it and an edger for around corners and windows, etc. Cut in first with a brush (e.g., where ceiling meets walls), but then roll roll roll. I'm sure there are videos to give you more specific guidance -- I just wanted to reassure you that This Can Be Done!