Addressing Invitations

GOOOOD Morning Guys!

I am sending out my engagement invitations in a few weeks.
I need a very basic how-to address invitations session. Here is what I would like to know:

1. How to address the outside
2. How to address the inner
3. Do I even have to address the inner
4. Who should get their own invite (like perhaps people over 18 living with their parents should get their own?)
5. Anything else I need to know that I am forgetting...

Thanks..I know the etiquette on this topic can be confusing!!!

S

Address the outer as

Mr & Mrs John Doe

for families I would put
The Doe Family

for single people:
Miss Jane Doe & Guest
(if the have a significat other put both names)
Miss Jane Doe & Mr John Smith

This is great!
Ok so how about inner...do i have to? If I have to, what do I write???