A paint's gloss is determined by the resins in the paint that bind it to the surface. A high-gloss paint surface is shiny, durable and resistant to water, and it provides an appropriate finish for areas that need frequent washing, like kitchen walls. You can paint over high-gloss oil-based paint on walls with another high-gloss oil paint with no special preparation. But to cover an extant high-gloss paint surface with a water-soluble latex paint, you'll need to prime the high-gloss surface.
Wash the walls thoroughly with a rag dipped in dish soap mixed in a bucket of warm water. Let the wall dry for two hours.
Remove all switch and outlet plates with a screwdriver.
Apply painter's tape to all areas of the wall that you will not be painting, including trim and the ceiling.
Apply primer paint to large sections of the walls with a roller if you're going to cover the walls with latex paint. Fill in the edges and corner areas of the walls with the paintbrush. Let the primer dry for two hours.
Apply latex or gloss paint to the large sections of the walls with a roller, then fill in the edges and corners with the same paint, using a paintbrush. Let the paint dry for two hours. Remove the painter's tape.
Remove surface mould from wall surfaces before painting with a solution of 1 part bleach to 4 parts water.