This is relatively simple: You should fill in all the docs on the website. But before, you should have found a laboratory, in SP, and you should convince the head of the lab to welcome you and your project. In my case, it has been an informal interview, in person, during a congress (Sept 2007 not 2008). As a post-doc, you have to write a project, which will go through several processes and several levels of assessments by professionals of the field. Then, after 6-8 months, a formal answer will be given for a two-years contract (extensible for one more year but you have to go through the same process (a bit shorter, only 3 months, but the answer can be "no")).