People usually here a song I did with someone or have heard my beats before, and they send me an email saying they'd like to do some work together.
So, you can start playing them shit from your catalog, or make something new for em.
Make sure you discuss payment FIRST.
I usually have MCs come to the lab, have a seat, play like 10 beats and let em pick one.
Then I take the cash, and buy some dinner.