According to Etiquette:
The Proper Way To Assemble and Mail Announcements
     You will have received:
            Announcements (Tri-fold announcements arrive flat; needs folding)
            Inside Envelopes (Slightly smaller envelope with no glue on the flap)
            Outside Envelopes (Larger envelope with glue on the flap)
1.         Assemble the parts of your announcement: Fold each announcement, accordion style, along the two score lines so the "Class of…" shows on the front (top), the verse is on the inside, and the name of the graduate shows out across the bottom of the announcement, when finished.  You may choose to put additional items inside each folded announcement such as; a senior picture(s), open house invitation, ceremony invitation, map to the open house and/or ceremony, confetti, etc. 
2.         Address the inside envelope: Use ‘informal’ names. The adults are listed first and children names are listed in order of age. Children over the age of sixteen should receive a separate invitation. If children’s names are omitted it infers they are not invited.   Examples:
                  Grandma         Uncle Bob, Aunt Susan, John, Sue and Tim
                  Bill and Helen                        Uncle Bob and Aunt Susan
3.         Address the outside envelope:  Handwritten with the ‘formal’ name and complete mailing address on the front using black ink. Do not use abbreviations except for titles and the state. Example:
Mr. and Mrs. William Jones
1554 South Wallace Street
Detroit, MI 48123
The return address is also hand written or a return-address label may be used instead. The return address should be placed on the back flap or the front, upper left corner of the envelop.
4.         Assemble the envelopes: Place the announcement into the smaller, inside envelop with the upper edge of the announcement inserted first and the announcement front (top) facing the flap side of the envelop (so when opened you immediately see the top of the announcement). A seal may be used to fasten the flap of the inside envelope. If not using a seal, leave this flap un-tucked. The inside envelope is then placed into the larger outside envelope with the name of the guests facing the back side of the outside envelope and the inside envelop flap facing the mailing address side (so when opened you immediately see the names of the invited guests). The outside envelope is then sealed using the glue of the flap and/or a seal. A seal is most often used on the outside envelop, though it may be used on the inside envelope, both envelops, or not at all. 
5.         Mail: For announcements used to also send an invitation to the ceremony and/or graduation party it is appropriate to put them in the mail one month (especially if inviting people from out of town), but not less than 2 weeks, before the date of the ceremony. If the announcements are just being used  only to ‘announce’ the accomplishment of graduation, it is still proper to send it up to two or three weeks ‘after’ the ceremony.