Firstly I read and re-read the script several times, trying to pin down the images it evokes in my head. Then I start breaking it down into panels. I draw little thumbnail images next to the text for each panel.
Then I'll do a layout where I figure out how the panels will fit together on the page.
Next I turn to the trusty internets and using image search engines and 3D warehouse, I'll gather everything I can that will either help my drawing or inspire me. In some cases I'll make crude 'sketches' of these images in Photoshop so I can get a sense on how these different elements will work with each other.
Finally, I'll break out the A4 size bristol boards and start drawing using several different pencils, pens, and brushes, to get the desired thick and thin lines.
I'll scan the speech bubbles later and spend several painstaking hours lettering using different fonts. Anyone know a letterer? Gah. And then there's coloring too, using watercolors and photoshop colors together. Coloring is much more fun. I do a base of digital colors, then I scan in several washes of watercolor and play with filters to see how they react with the digital colors underneath.
And now letters! Hand drawn speech bubbles, scanned in, and a font thrown inside...