Hello Nora, welcome to the forum.

To answer your questions,

1. Usually sending a picture with your request is a good thing to do. Some celebrities have photo's they send themselves but some don't, so best to be sure and include one yourself.
2. Tell her a little bit about yourself, tell her what you like about them. Favorite show/episode/storyline. Stuff like that. Somewhere in your letter ask them politely to sign your picture.
3. You put the picture of the celebrity, your letter, IRC's (I would recommend 2) and a return envelope with your address already on it inside a bigger envelope. On the bigger envelope you put regular postage like you would on any other letters you would send.
4. That's all, then the waiting game starts. ;)

Not sure about successes from those particular people you asked about, but if you use the search fanmail.biz function at the top of your screen, enter the name of the persons you want to write to, it will show any previous successes with addresses you can use.

Good luck with your requests.