You create a Subcontractor the same way you create an Employee:
Click "Employees" in the main menu and click the button to add a new Employee.
The important step in creating a Subcontractor is ticking the checkbox labeled "CIS Subcontractor"
If you've already created the Employee record then you can access this checkbox under the "Employment" tab of the Employee record
Important: Once you've started paying an employee, you cannot change this checkbox
When you mark an Employee as a Sub-Contractor certain fields and options are hidden (i.e. those related to Leave, Pensions and Company Director) and new options are shown, such as those to set the Subcontractor Type, their UTR and so on.