Copy the portrait with a color copier. Use the ruler and pencil to draw a grid with 1-inch by 1-inch squares over the entire portrait. You can use larger squares, but the painting may not be as detailed or accurate.
Brush three to four coats of gesso on the stretched canvas, allowing to dry between each coat. Even on pre-gessoed canvas, this is important so that the surface is as smooth as possible.
Draw a grid proportional to the grid on your portrait using the ruler and chalk. If the canvas is the same size as the portrait, the squares should be 1 inch by 1 inch. If it is twice the size, the squares should be 2 inches by 2 inches.
Start at the top corner, and sketch the details in each square of the portrait into the corresponding square on the canvas. Use the chalk for this. This is the most important part of the process, so take your time and draw carefully.
Dip a paint brush into acrylic paint, and fill in the sketch. This can also be done with oil or watercolor paints, but the opacity and quick-drying characteristics of acrylic paints make them particularly suitable for novice painters.
Decide if you want to paint the portrait from the top to the bottom, or one color at a time. Most painters prefer one way over the other. Fill in the sketch with paint. Clean your brushes with water between each color.
Allow to dry overnight to ensure it is completely cured before handling. Most acrylic paintings will dry in less time, but it varies based on the body and thickness of the paint.