Here are some links you may find helpful in formatting your address list.  You can decide just how formal or informal the addresses will be, depending on the formality of your occasion.  Even though we're often less formal these days, it's always a good idea to respect how your guests would like to be addressed.  Remember, I'll pen the addresses exactly as you send them on your list (i.e. 'Great Aunt Rose' or 'Mrs. Rose Sherman') so please be sure your list is exactly the way you want it to look on your envelopes, and double-check spelling, etiquette, and addresses.  

Emily Post's Guide to Addressing Correspondence

The Knot - How to Address Wedding Invitations

Crane & Co - How to Address Your Envelopes