I didn't get very good instructions with my invitations on how to assemble them. Please advise!
Your invitations may already be folded the right way (if flat cards, then no worries), but if not, you should fold them, probably in two, like a book. The fold should be on the left, with the invitation text on top and the inside page blank.
Enclosures such as maps are usually placed beneath the response card if the card is placed on top of the invite, or they can be tucked inside the invitation if it's a folded card. The map can certainly be bigger than the response card. You can have the map printed on a matching card, but many couples simply copy it onto an 8" x 11" piece of paper.
If a sheet of tissue paper came on top of each invitation, it's your choice whether to leave it in or not. It's there to keep the ink from smudging (while most inks used these days won't smudge, the tradition has continued).
Everything is inserted in the inner envelope. The text of the invite should be facing the back of the envelope, so guests will see it when they open it up.
The unsealed inner envelope gets inserted into the outer envelope (this is starting to sound like surgery) so that the handwritten guests' names you've put on the inner envelope are visible when they tear the thing open on the way from their mailbox.
Wedding Invitations + Wedding Stationery