I don't have a tutorial but I recently made a Lego person cake. what I did was started with a flat "base body" shaped cut out of fondant and then cut all the parts out of fondant and attached to to the base piece of fondant. The cake is in my photos, the quality of the pictures is horrible but you can look at them to get an idea.
You can find one that is really helpful here
http://dazeycakes.blogspot.com/
scroll down and you'll see it
On what size you want. If it small(topper for a cake), I would take a real lego guy and take him apart. Then copy those shapes in fondant/gumpaste mix, modeling chocolate. Then you could paint, let dry and attach just like the real mcCoy. Otherwise, if your figure is larger, you could use the same above but internal supports(pipe, pvc etc.- this gets more complicated) for a form and mold the above on.
I have also made larger figures in rice krispee treats(rice krispees pulvarized so treats are smoother), then cover is fondant. Same method for parts reproduction9mentioned above).
If it is cake- carved or 3-D, gets alot more complicated. I am sure others here could give you suggestions if that is the direction you are going.
Let us know when you finish it with a pic!
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