Prepare
Unhang, strip the hardware, sand
Lift the door off its hinges and lay it across two trestles: flat, the paint cannot run and you can see what you are doing. Remove the handle, roses and latch plate rather than masking them. Sand the whole surface with 180 grit to dull the old paint, or 120 on a bare door, then wipe down with a damp cloth.
A bare or MDF door gets our Multi-Surface Primer first, in the tint matched to the final colour; a varnished door must be sanded until every trace of shine has gone. Fill chips with wood filler, sand again, and check the edges: they are what rubs and chips first.