To send your finished mail, you should have the following:
2 SASEs – You’ll need two ‘self-addressed-stamped-envelopes’ (SASEs). This means that you have one envelope that you’re going to send all your stuff in, and this should be stamped and ready to go with the address you’re sending to on it. The second one is mailed with all the other content. This second one should also be stamped with the correct amount of postage stamps, and (*important*) it should be self-addressed, as in this envelope will be used to mail stuff back to you if you have sent a photograph to sign. (If you haven’t included anything extra, and don’t expect to receive anything back, you don’t need the second SASE)
If you want to include a photograph with your letter, I would advise getting a manila envelope size. Also, if you don’t want the photo to bend, get a light piece of cardboard and stick it in there too!
The Correct Amount of Postage Stamps – This should be self-explanatory, but keep in mind that if you’re sending out-of-country, you should be able to adjust for that. When that happens, I would just take it to the post office and mail it off manually from there.
Your Letter, Photograph, other content (etc.) – This is also self-explanatory, haha.