I'd start by just sketching them in. Don't worry about nice edges or shading, just get the basic lines down so you can see if the width and spacing is consistent. Then step back and see what needs to be adjusted. When it comes to the finer detail work (shading/highlighting, straight edges, etc) I'd start with the lighter color. That way if you're a bit sloppy with the application and creep into the other colors area, it's easier to paint over a light color with a dark one. In the end, I'll off take several passes back and forth between the two colors to fix any lines that need a bit more work.
If you find the straight lines/edges are giving you trouble, you can always paint one color and then use painters tape to create a temporary mask for the lines while you do the second color. You can often find very thin strips (or cut your own) at art stores and it's easier to use on a larger figure like this.
