Step1
First, go online and Google on the name of the person you want to send to. Try to find their listed contact information, probably through their company. Most celebrities of any kind want things sent "care of" someone else. Use these channels or your package will probably be immediately thrown out.
Step2
Once the "care of" address is found, use your word processor to write a decent business-format letter explaining who you are and why you're sending this. A business format letter has your name, company, and address at the very top, two line break, their name, company, and address, two more spaces, then "Dear So and So," then the body of the letter. It can be Googled with the keywords "business letter format".
Step3
When writing the letter, keep your tone professional, no matter why you're sending the gift. Even if you're sending it out of sheer appreciation for the celebrity in question, do not gush on about how much you love them, or any other strangeness. "Dear Mr./Ms. xxxxx, I appreciate your work to no end, you have inspired me to acheive great things in my own life, and I wanted to send you this gift as a token of my appreciation," works so much better than writing as if you were a friend of theirs. You're not. End it with "Sincerely," or "Regards" and your signature. Do not end it with "Love."
Step4
Spell and grammar check your letter. Always. Then have someone else read it, just in case the computer missed anything.
Step5
Package it all up together, make sure both the send and return addresses are correct, and take it to the post office. The US Postal Service has a new rule that if your package is over thirteen ounces you have to take it in and hand it to someone personally, so be aware of that.
Step6
You probably won't get a response, but if you're going to, it will probably be six months later. Don't hold your breath, and don't send anything perishable.