Hmm are you having trouble with implementing an ACL or with determing your ACL?
If it's syntax, the ? should help you out, as far as determining your mask goes just keep the reverse masking in mind and you should come out okay
If you're a bit more specific I'll take a crack at it.
(Btw, search the forums, I recall there was a gem of a thread on ACL's with tips etc, but it's probably pretty far down). The site is also searchable through google, just query for whatever you want and add a 'site:firewall.cx' to the end of your google query.