I'd like mine to look like the set on the GW website but I was wondering what is the best way to paint the tartan or would it be easier if I commissioned them? Bare in mind I've only started painting recently and my limit has been WOMT which aren't too difficult.

http://www.jenova.dk
Check out this website. Jenova has an article regarding how to paint a tartan

I think the key with Tartan (and other intricate patterns) is not so much to actually reproduce the patterning faithfully, but instead a simplified version that produces the desired effect at about arm's length, or when viewing the mini on the table.

If you were to somehow actually paint a tartan on the model, it would be so teeny tiny that you would hardly see anything, and instead of looking like tartan it would just look muddled and too busy. More likely than not, unless you're a Golden Demon painter, it is also just going to be too difficult to paint a good tartan exactly as they look in real life.

Jenova's tutorial is a pretty good, standard way of getting tartan down. Another option, if you're up to it, so to go for a little more of a woven/hatched effect, where the lines appear to alternately go over and under one another.

Lastly, you can paint broad intersecting stripes in one color, and a very fine stripe down the middle of those broad stripes in another contrasting color.

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Well, that's my 2 cents.