Spraying would probably give the most "flat, stroke-free" finish, but requires special tools and materials and a considerable amount of skill/experience for success.
With brushes (I omit rollers for a reason as I never use a roller on trim, baseboards, doors, stair rails, spindles) you can probably get acceptable results with skills you already possess to some degree. The key things are:
- preparation- thoroughly de-gloss and clean the existing surface of all dust. Mask off and tarp over adjacent areas in case of drips/spills.
- brushes- you can get a smooth stroke-free finish if you commit to using only the very best quality brushes. Yes they are expensive, yes it will be worth it (clean them religiously and store them properly and they will last for years).
- paint- use quality paint.
- technique- don't be stingy with the paint and always maintain a wet edge. Start with less conspicuous areas and your skills will be honed by the time you get to the important stuff.
- patience- the final appearance is directly proportional to the time you spend making it perfect.