Help with 1999 Nissan Sentra keyless remote programming

I am new to this great forum website and I could use some help. My wife had her purse stolen at a store recenlty with her keys and remote. I bought a used remote for her 1999 Nissan Sentra but now I need help programming it. I don't even want to think of a dealer unless it's a last resort. Can anyone guide me through programming a 1999 Nissan Sentra keyless remote? thank you in advance - emil