I'm currently doing this with coarse sand paper but how rough does it have
to be?

Not very. Just get the shine off it.

Do I have to seal it with PVA, use a wood primer/undercoat first and
then use the kitchen emulsion paint that I am wanting to use?

I would use an acrylic undercoat, like Glidden Primecoat Acrylic Wood
Primer/Undercoat. Then almost any paint at all on top of that, including the
kitchen emulsion.

Christian.