* Buy an envelope large enough to hold the photo you wish to have sent to you. If you are sending your own photo to be signed, then just test the envelope to make sure there's plenty of room for it.
* Write your name and address on the outside of the envelope as the addressee.
* Put enough stamps on the envelope to cover the shipping of the envelope with the photo inside it to you.
That is a self-addressed, stamped envelope.
Take the photo, the SASE and...your fan letter or whatever else you're sending....inside a larger envelope. Address the larger envelope to the celebrity and put enough postage on it to cover sending everything inside to that person.