Lay the fabric that will be used to paint the background on flat on the floor. The fabric will likely be in the shape of a rectangle laying on its long face. This is because most stages are oriented in a rectangular shape.
Draw a rectangle 50 percent of the size of the fabric in the center of it with chalk. Connect the corners of the rectangle to the corners of the fabric with a chalk line. This will give you a good grid for scaling.
Draw nine to 12 equal sized boxes on the fabric using a differently colored pieces of chalk, creating a grid pattern.
Draw a grid of the same scale on the model photo. Each grid on the photo will correspond to the same grid on the fabric. Drawing the background this way will help ensure accuracy and scale.
Copy each grid to the fabric using painting supplies. High detail is not necessary because it will not be visible to the audience. Use softer colors that do not contrast heavily. The backdrop should not make the background pop out, but rather be part of the atmosphere.