Your invitation order should include either a small guide, or a pre-assembled invitation for you to examine.  In case not, here's what to do:

1. Address the mailing envelopes with the invitees' full address and the return address, if necessary (on many invitations, you can order the return address pre-printed on the outer envelope).
2. Address the inner envelopes with the invitees' names only.
3. Place the invitation face-up on a table.
4. Place a piece of tissue over the imprint area.
5. Place the reception card (if any) face up, on top of the tissue.
6. Tuck the response card face up under the flap of the response envelope, and place on top of the reception card (or tissue) with the response card facing up. Don't forget to put a stamp on the response envelope.
7. Place the entire pile inside the inner envelope.  The back of the invitation should rest on the inside of the flap as you insert the pile.  Fold the flap, but do not seal the inner envelope.
8. Place the inner envelope inside the mailing envelope.  The names on the inner envelope should face the back of the mailing envelope, so that when the mailing envelope is opened, the invitees will see their names.  Seal and stamp the mailing envelope.  You should take one invitation to the Post Office and have it metered so you can apply proper postage.  Fully assembled invitations are relatively heavy, and odd shapes may also require extra postage.

There will be some variations depending on how traditional or non-traditional your invitations are.  The main point is that your guests should see their names on the inner envelope as the mailing envelope is opened, and should read the response card when the inner envelope is opened.  The image below is courtesy of Birchcraft Studios: