There is no step by step howto. There is a book that tries to explain some policy writing but it is a bit outdated and it costs money (http://www.selinuxbyexample.com
I guess i could help you write a policy, and although that may give you some insight, do not expect that it will make everything clear about selinux.
If you want some guidance, you can come see me on irc (irc.freenode.org channel #fedora-selinux) my nick name there is dgrift
policy writing in itself is not so hard but it requiires knowledge about security, computer security and the linux system. also it requires that you know the basics and not so basics of selinux.