I think, you have read my previous two posts related to implementing forms authentication in report server.
Sql server reporting server and forms authentication – Part 1 – Understanding the concept.
Sql Server reporting server an forms authentication – Part 2 – How to implement.
I just followed Russell’s article to link sql server report server to aspnetdb. So, I prefer to redirect my users to the same post. I don't want to repeat the whole story here again. So, please go here and continue your reading.
Hope with this post, you are completely done how to use, implement and linking the default aspnetdb to the report server in forms authentication.