In the usual way - an offer is made and accepted, and whatever has been agreed previously forms part of the contractual agreement.
Common pitfalls include:
- Carelessly sending email messages which have subsequent contractual implications.
- Publishing information on your website which later forms part of contractual agreements without your wishing it to do so.
- Poor website design allowing contractual obligations to be created without you wishing them to, or contracts which are not covered by the terms you would like.