I want to make two employees redundant and use subcontract labour when I have work. How do I do this?

This is an extremely complicated area and not as straightforward as you hope. There is no quick answer to this and it needs careful handling at every stage. What you can do will depend very much on whether or not the work has ceased or just reduced. There is also no guarantee that anyone you use on a subcontracting basis will not be classified in practice as an employee.
 
What you are looking at here is a redundancy situation and you need to consult with your employees about the options if you no longer have enough work to guarantee work to either or both of them. You may find that one or both of them would be prepared to reduce their contracted hours to accommodate the reduction in work and work overtime during the busy periods. If, at the end of a meaningful consultation with your employees, you find that you have no alternative but to make them redundant then you can look at dismissal on those grounds. This way you will be better placed to defend, if not avoid, a claim for unfair dismissal.