Suzie Creamcheeze shows you how with FOSS.
Suzie has been working entirely with FOSS tools – Free and Open-Source Software. She has detailed every part of her process, including many of the planning stages I also emphasize. You can see her script, storyboards, animatic, production cels, timing notes, and the final.
One point I emphasize in the “Get Animated!” book is that the traditional screenplay format is not only unnecessary for good animation, it probably hurts the process. This is not an original view on the subject! In fact most people who know something about animation tend to agree.