Tracing is one way. The way that I came across and pursued, was with the aid of using a grid. It involved placing a grid (drawn on a plastic protective sheet) over the drawing, and then drawing what was in each individual square. You can also turn the original drawing upside down, to help your mind think in replicating by actual sight, instead of what you "think" it should be.
Another variation of this method, is to place dots at various points to replicate the main design, and then quite simply, connect the dots.
Another method that I liked was drawing on graph paper. I could count the little squares to line things up and to get things be the same size, or not.
Anyway that you practice, helps to teach the brain the method that you are using, and the proportions too.