Sketch several pencil studies of the original piece. Study the proportions of the original, as well as the technique of the original master.
Choose the medium you'll be working with. Match the medium to the type of medium used in the original masterpiece. If the artist did the painting in watercolor, work with watercolor. If the artist did the painting in oil, do the painting in oil.
Place a high-resolution color photocopy of the painting you are replicating in a position where it is clearly visible. Place the canvas or paper on your easel at a height that is conducive to painting.
Draw onto the canvas or paper (if this will be done in watercolor) the basic forms and design of the painting. This should be done lightly and without inclusion of too much detail. Practice reproducing the colors you see in the original painting. Do this on the palette, and mix the colors with the paint brush.
Paint the first washes of color. Cover the entire picture plane. Work deliberately, with brush strokes of a similar style to the painting you are reproducing. Although the painting you are copying may have thick, textured brush strokes (like a Van Gogh), most artists start with a thin layer of paint and build up from there. At regular intervals, try turning the painting and photograph upside down and compare them.