How do you make a custom?

Alright for some reason my previous post didn't have the text.

So my buying the MkIII with lascutter and I want to turn him into a Iron warrior and not a imperial fist. How does one go at repainting these figures

Alright for some reason my previous post didn't have the text.

So my buying the MkIII with lascutter and I want to turn him into a Iron warrior and not a imperial fist. How does one go at repainting these figures