Here's the [rather tedious] process I went through to paint my Nightwind of Aldrassil image (you can see the final version here: [link]
I think this might be more useful as an example of what NOT to do than what TO do, but it might at least be insightful and a little entertaining, I hope.
(The painting was done in 2006, but these images have sat around on my desktop since then, and I only just crafted them into a tutorial. I hope I didn't leave out important details!)
If you have any questions, I'll do my best to answer them!