This post from Richrap is highly recommended by one of our customers who found it invaluable in his build:
I have been using Slic3r for a few months now, it's coming along well and deserves lots more coverage, so here are a few notes and a very short guide to using it and what the settings mean - nothing difficult as it's already very easy to use and understand.
In this Part 1 we will look at getting Slic3r running, calibrate the extruder and take a look at the Print settings.
Part 2 will setup the printer, filament and print speeds along with doing some printing.
Part 3 will experiment with low layer heights, hollow printing and any other features or limitations discovered.