By finding enough candidates for enough businesses willing to pay the huge finders' fees that recruiting firms demand.
One of the things that amazes me is that companies will pay a recruiting firm tens of thousands of dollars to find a candidate that makes it through the interview process and stays there for at least 90 days but then they only offer $500 to their employees for referrals that get hired. I pointed out to HR at one company that I worked for that one of their recruiting partners pays $1000 for a referral that gets hired; given how much they would pay a recruiter, they should be offering at least $5,000 referral bonuses to someone recommends a candidate they hire, whether the person making the recommendation is an employee or not. They decided to increase the referral bonus to match the recruiting company (I kept feeding referrals for my employer through said recruiting agency -- my relationship with them started before and lasts long after being with that employer).