All NEC3 contracts deliberately do not specify how they can come into existence. That is because they are designed to be used worldwide and therefore how they are made will depend upon the applicable law.
It is entirely up to the parties as to how their contract comes into existence and what form it takes. It can be a simple contract, which can come into existence in many different ways in the UK: being signed by both parties, and by offer and acceptance. It can also come into existence by the use of a specialty contract such as a deed, in which case care will need to be taken as to its form.
If you are unsure about how to make such a contract we recommend that you get some legal advice on it.