What tools should be kept in mind while and before making a contract? What things are essential?

Contracts should capture A) how the things work in as-usual way and should also include B) agreed understanding as to how things would work if things are not as-usual.

So, on one hand it captures agreed mechanism for performance between the parties on the other hand it captures risk matrix to explain who takes how much risk.

The important part to take care here is to ensure absence of ambiguities and coverage of all possible risks that can be thought of during the process.