Wedding invitations are assembled in size order similar to a wedding cake with the largest layer on the bottom and the smallest layer on top. When stuffing your invitation set, start with the invitation first as the bottom layer. Next, tuck the reply card underneath the flap of the reply envelope and set this on top of the invitation. If you have a reception card, this would then be place on top of the reply card and the set would be placed inside the envelope. If you have additional insert cards, assemble them according to size relative to the other inserts.
To ensure you have stuffed your invitation the proper way, remove the invitation from the envelope with your right hand. If the insert cards sit on top of the invitation and you can read the invitation without turning it, you know you have stuffed correctly.