Practice!!!
My experience is not that complicated a pattern, but I freehanded a rampant lion onto the right shoulderpad of most of my space marines. It\'s amazing what repetition will do.
What\'s my point? Well, you could just freehand the picture several times (if you are cautious, do it on paper a few times first) and when you are happy with the result, you can repeat it. You\'ll be surprised how consistent you get. Not perfectly, but with my marines it\'s all supposed to be hand done anyway so minor variations are expected.