To engage a someone via a contract for services, you will need to request this via the PO request form. You will most likely need to ask them to complete a new supplier form, which is linked on the PO form, and you can see the PO requesting page for further information.
If they are UK-based and self-employed, you will also need to complete an employment status checklist. Please see the Payroll webpage for more information. This check isn't necessary for accommodation.