A Contract of Employment is the formalisation of the arrangement you’ve agreed to for a person to join your business and the legal foundation for your employment relationship. Given how important work is, you should take the employment contract seriously, and try to ensure that the offer process is also a positive experience for the person you’re to which you offer employment.
If you want to keep the contract very straightforward, you can have a short, clear contract. You need to provide a formal written offer (or follow up on a verbal offer with a written copy), then give the candidate time to consider the offer. The individual should then accept the offer formally, usually with a signature.
It is possible to change a contract but only by mutual agreement.