You could do a lot worse than to spend some time at Defoe artist Leigh Gallagher's fantastic blog (you'll find it here.) It's an absolute treasure trove of goodness with wonderful 'How to's' on each of his fantastic covers complete with prelims, pencils, inks, reference photos and more - sheesh!
Above is the cover of the Defoe 1666 graphic novel (buy it cheap here) and there's a typically excellent walkthrough of how Leigh 'made' the cover on his blog.
Below are all of Leigh's 2000AD covers to date and again, each has it's own 'how to' on his blog. A huge thanks to him for sharing them, and for the little glimpses of life in Tharg's Nerve Centre!