For the most part, wedding invitations are assembled in size order. The invitation itself is first. The enclosure cards are stacked on top of the invitations, not inside. The reception card is placed on top of the invitation. Then the reply envelope is placed face down on the reception card. The reply card is slipped face up beneath the flap of the reply envelope. These are the most frequently used enclosures. Any other enclosures are added face up in size order (usually at-home card, direction card, accommodation card, pew card, etc.).
The single-fold invitation and its enclosures are placed into the inside envelope with the fold of the invitation at the bottom of the envelope and the engraving facing the back of the envelope. You can tell whether or not you stuffed the envelope correctly by removing the invitation with your right hand. If you can read the invitation without turning it, it was stuffed correctly.
The procedure for assembling traditional invitations (those with a second fold) is similar. The enclosures are placed on top of the lower half of the invitation's face in the same order described above. The invitation is folded from top to bottom over the enclosures. The invitation is then placed into the inside envelope with the fold toward the bottom of the envelope. As with other invitations, traditional invitations are correctly stuffed when they can be read without being turned, after being removed from the envelope with your right hand.
Once stuffed, the inside envelopes are inserted into the outside envelopes. The front of the inside envelope faces the back of the outside envelope.