This should be completely dry.
Sand down lightly to smooth the grain. Also sand the edges of timber as paint has a tendancy to 'pull back' from angles as it dries.
Dust down using an old paintbrush or dusting brush.
Apply knotting solution to any knots. If you intend to varnish the wood, do not apply knotting as this will show in the final finish.
Apply a coat of suitable primer using a brush. Brush application is preferable as it tends to work the primer into the surface more efficiently. For sealing surfaces to be varnished, refer to varnishing section
Video: Preparing and priming new wood
Providing that the old paintwork is in sound condition, there is no need to remove it. It can actually form a very good base for new paint.
If there are lots of defects in the paint - chips, lumps and bumps, badly painted or beginning to flake, craze or blister - it should be stripped.
Sound paintwork should be washed with sugar soap and rubbed down to provide a smooth surface ready to receive the new paint.