Optus have a pretty extensive how to for email.
Just a word to think twice before using an ISP email account (any ISP email account). These accounts are generally less feature rich than dedicated online email options (security, spam detection, interface, accessibility, etc.) but more importantly they are not portable. If you ever decide to leave Optus you will lose your email account. You might want to consider setting up a free online account like GMail or Outlook etc. That you can access anytime and will work with any ISP.
Thanks for your input, Peter!
@caseymaz - I'd recommend following the 'How To' guide Peter has provided above. If you have any further questions let us know.