First, you sign up on a forex rebate website (there is no cost to do this). Then, you look through the brokers that partner with that site. Choose one or more that interest you, and then open accounts on their sites. This too is free, but you will need to make a deposit before you can trade.
Once you have done that, you will begin earning rebates as you trade. You will receive them as either discount on commissions or spreads or as regular payments from the rebate provider on a predetermined schedule.