Painting skirting is one of those jobs that looks easy and goes wrong fast if you skip the prep. Get the groundwork right and the finish does itself.
Prep first. Wipe everything down with sugar soap to lift years of dust, scuff marks and that greasy film that builds up at floor level. Let it dry, then sand the whole length with 180-grit — you're not stripping it, just keying the surface so the new paint grips. Hoover the dust, then wipe with a tack cloth or barely-damp lint-free rag. Skip this and your topcoat will peel.
Fill and caulk. Fill any dents and nail holes with a fine surface filler, sand flush once dry. Then run a thin bead of decorator's caulk along the top edge where the skirting meets the wall — smooth it with a wet finger. This gives you that crisp, gap-free line and is the single biggest difference between a DIY job and a trade finish.
Prime if needed. Bare or knotty wood needs Zinsser BIN to lock down resin and stop knots bleeding through. Old glossy paint that you've sanded back is usually fine to go straight over with a quality topcoat — but if it's slick, an all-surface primer buys you insurance.
Topcoat. Two thin coats beat one thick one every time. For durability and a beautiful finish I'd reach for Little Greene Intelligent Eggshell or Farrow & Ball Modern Eggshell — both wash well and resist scuffs. If you want a tougher, harder-wearing sheen, Dulux Heritage Eggshell is cracking on trim. White-wise, Slipper Satin or Pointing keep things soft and warm rather than the harsh brilliant-white most builders default to.
Use a 1.5–2 inch angled synthetic brush, lay the paint on along the grain, and tip off in long strokes to avoid brush marks. FrogTape along the floor if your carpet's exposed.
Give coat one a full 16 hours to dry before the second — skirting takes knocks, and rushing it means a soft finish that marks.