Paint the entire wall in a matt emulsion of your choice and allow it to dry. Use a tape measure and spirit level to measure out horizontal lines, lightly marking guide dots on the wall as you go along.
Run masking tape above the lines and apply a coat of matt paint over the tape to prevent the gloss creeping under, and allow to dry. Use a roller or brush to apply two coats of a quick-drying gloss paint. Once dry, remove the tape to reveal your new look.