Hello Carla !
I will try to help you. I have been doing this for years.
You are right to put an envelope into another one, it's easier to get an answer. Both must have stamps.
Generally I put two stamps on each envelopes, I add more if I see that the weight is more heavy or if the mail is a little domed.
I don't know which country you come from, but in my country, we have international stamps so I'm using those.
I tried to buy American stamps on USPS website but they do not ship outside the United States.
I always send pictures in the mail, it's better if you want to received signed pictures.
I print them with my own printer and photo paper for a better quality.
I do not send many pictures, only 3 or 4 (no pictures they post on their social networks) if it's an actor you can send pictures from movies or TV shows, even promotional pictures.
I do not have any special tips, ask nicely and be respectful.
If you do not get an answer, do not take it personally. Sometimes they are busy / not in the mood, doing promotion for their work, or they have changed of agency.
They also have an agenda, friends and family so they might not answer right away. Be patient
If you have any questions, do not hesitate. I will try to help you if I can.
Good Luck and I hope you will get an answer.