You give them some work and then you pay them, simple as.
However; you will have to PROVE that they are self-employed by making sure you personally are not the only person who pays them to do a job. They need to have at least TWO people paying them. This is how a lot of agencies work, the client pays the staff the hourly rate and pays the agency the agency-fee seperatly.
There are also other complications, like if the staff use your equipment and uniforms, as this may tip the balance in the favour of the 'employed' status as opposed to 'self employed' .