(water) based paint over existing oil based paint, you must use an oil based sealer undercoat. Follow this with two coats of (water) based paint to avoid the paint peeling off at a later stage.

Wash brush in water or turps as per instructions on the paint can. Rinse in soapy then clear water. Hang up to let bristles dry.

Measure the square meters of the area to be painted, the formula is width x length = sqm. A wall 2 meters high x 4 meters wide = 8 square meters. Each manufacture lists the coverage of their paint per litre on the side of the can.