It varies by company, some use free lancers some write in house. Some want a hard copy some will take electronic submissions, etc. Pick up a copy of The Writers Market for the new year, it will give you step by step instructions as well as telling you who is in the market, if and how they accept submissions, what they pay ( probably no more than 25 dollars per item purchased,) how to approach them and the types of cards each company is liable to purchase - funny, romantic, dirty, etc.
Good luck.