This is one of our most common questions, and stuffing invitations is a process that takes some time. If your invitations were addressed by hand, you will want to check over them first to ensure no one was inadvertently omitted and that the address is correct.
Apply postage to your reply envelopes, then nestle the reply card underneath the reply envelope flap. Then the invitations are stacked in size order, with the largest piece on the bottom and smallest on top. Tissue is often used to separate each piece if desired, even though most often it is not needed. The exception is when digital printing is used as it could possibly bleed onto a facing card due to the pressure applied during the mailing process. Many years ago, there was a very popular wedding stamp with a small red heart in the corner. It was a bride-favorite until people started receiving invitations with a faint red heart on the beautiful wedding invitation itself. The heart had bled from the stamp on the reply envelope that was facing it - ugh!
Take care to stack everything neatly, grab it as a unit and stuff into the inner envelope so that everything is face up when you open the envelope and remove the invitation with your right hand. Then the inner envelope with the names on it is inserted into the outer envelope so that the names face the back of the envelope - or the opposite way from the outer envelope. You want the recipient to see their names on the inner envelope when the open the outer envelope.