i am a profesional painter, but not a professional ski painter.
the 2 most important things will be
-sand lightly but thouroughly (evrywhere) to break the sheen, any fine grit sandpaper will work.
and
-use durable paint. i sugest oil based of some sort (dries much harder, way stickier) you might also want to do some reaserch and find some paint that is desinged to withstand cold temperatures without peeling/flakeing/cracking/checking
primer coat aint a bad idea although i doubt its nescesarry if you use oil paint and as long as your not sanding down to bare wood core. wouldnt hurt though.
also
people have been saying to use spray paint, theres nothing wrong with this but i would suggest using a roller for at least the 1st coat (fine finish roller, you could use a small one, known as a wizz roller) this will help the first coat penetrate into the tiny grooves you have made by sanding it (this is called backrolling in the industry). you can just spray the skis then roll them, so you dont also need like another setup for the roller, just run the dry roller over the skis after spraying them while theyre still wet.
and
clear coat will make inevitable chips/scratches from edges less noticible.
i have never painted skis so all of the above is not to be trusted.