Oh they are so easy really
First you have to figure out what size you want to print them at, I think mine will be 5x8 invitations then
Go to Page Settings and change the size to height of 5 and length of 8 (or whatever size you choose)
and then choose what sorta way you want yours to be set up, I will show an example of the draft of mine
If you need any more help, don't hesitate to ask, I would be more than glad to help