The smooth finish lives in the prep, mate — not the paint. Most streaky, ridged woodwork comes from skipping the boring bits.
Start by filling, then sand the bare wood to 120–150 grit. Always prime — a bare or patchy surface drinks paint unevenly and shows it. For new or knotty timber, Zinsser BIN (shellac-based) seals knots and stops resin bleed; for previously glossed or tannin-prone wood, Zinsser Cover Stain. Sand the primer flat with 240 grit once dry.
Now the golden rule: sand between every coat. A quick rub with 320–400 grit knocks back any nibs and gives the next coat something to grip. Vacuum, then wipe down with a tack cloth or a barely-damp microfibre — dust is the enemy of smooth.
For the topcoat, modern water-based trim paints have come on leaps. Little Greene Intelligent Eggshell and Farrow & Ball Modern Eggshell both flow well and self-level beautifully. If you want a tougher, slightly more lacquer-like finish, Dulux Heritage Eggshell is excellent. Two thin coats always beat one thick one.
Use a proper synthetic brush (Wooster or Purdy) or, for flat panels and doors, a high-density foam roller followed immediately by laying off lightly with the brush — that's the "roll and tip" method and it kills roller stipple.
The technique that matters: load the brush, spread the paint, then do one final pass in long strokes from a wet edge to a dry one, finishing in the direction of the grain. Don't overwork it — water-based paint sets fast and fussing leaves brush marks.
Keep the room warm but not hot, and don't paint in direct sun or a draught — paint that skins too quickly traps brush marks before it can level.
Colour-wise, a soft off-white like Farrow & Ball Pointing or Little Greene French Grey - Pale keeps trim crisp without going stark.
If you only change one habit: sand between coats. That single step is what separates DIY from a sprayed look.