Find out the location of the billboard, the amount of time it takes to draw it and the weather patterns of the locality. Obtain a sketch or picture you need to paint, and calculate the magnification ratio you want to apply; for example, how many times do you want to increase the size of the artwork from the sketch or drawing to the billboard size.
List the number of colors you are going to use and the types of the paint, as well as the brush and the medium -- whether "oil on canvas," "oil on tarpaulin" or "oil on wood." Determine their costs and choose your working day and time.
Wear protective gloves and clean the surface of the tarpaulin you want to work on, using thinner on a cloth to remove any oils and other obstacles. Erect the billboard with the help of other people or set up your ladder to the level of your work area. Mix your colors with the thinner. Paint an undercoat according to your design and let it dry.
Mount your paints to a vantage position then mark a point on your painting surface using a brush and a little dull paint. This point will act as the background source of all the artwork contours and dimensions, creating a vanishing point.
Paint the outlines of the farthest background features and shade the outlines and contours, gradually adding more colors. Change your brush and color if need be, and work your painting toward the foreground, starting with the outlines.
Take a short break and observe the billboard from a distance. Countercheck from the sketch or drawing if the features are in tandem with the ones on the billboard. Add in details again by intensifying the color, starting with the farthest background and coming toward the front. Clear any mistakes using the cloth and turpentine.
Let the painting dry then add more color if need be, taking caution not to apply too much paint and causing it to run. Clean your work area and allow the painting to dry naturally. Blow the painting using an air dryer in case there is a threat of changing weather, like strong winds, dust or rain.