The contract of employment is a legally binding document so think carefully about the content in terms of the obligations it puts on you.
If you write it in there, you have to be prepared to do it. Consider carefully the nature of the work people do for you; do they work full-time or do you simply offer them work as and when you have it? Can they turn down the work you offer them?
These are important points which will dictate the kind of contractual documentation you give them.
What if I directly employ them?
Where your staff are employees or workers, you must give them a written set of main terms by the time they start work, regardless of how many hours they work.
What about other policies?
Think about what other policies you want to include to protect you and give you flexibility (e.g. recovering wages for breakages; company vehicle rules; uniform policy; overtime policy).
Without including these in the contract, your staff will not have a record of what your expectations are and you may struggle to enforce them.
What if I want to amend my terms?
Contracts of employment can be amended but because it is a legally binding document, changes generally need to be agreed with the employees.