That depends on whether you are the one referring or being referred to the site, so we've broken it down in general terms for both:
Referrer -
This varies from betting site to betting site, but generally you will be able to claim the refer a friend bonus after you send your unique referral code to a friend, and they successfully sign up, deposit, or place a bet for the first time. Make sure you send them through one of the links on this site so they don't miss out on a their welcome bonus too!
Referee -
Once you get your hands on a referral code or link from a friend, all you need to do is register at the betting site (click on any of the numerous links on this site that will point you in the right direction for an awesome welcome bonus too), and fulfill the qualifying requirements. This can range from just registering, to putting down a certain deposit amount or placing a wager above a certain amount.