Writing a smart contract involves creating a set of rules and conditions that will be executed automatically on a Blockchain platform. Before writing a smart contract, one needs to determine the purpose, scope, and requirements of the contract. Next, choose a Blockchain platform and programming language suitable for writing the smart contract. Popular options include Ethereum and Solidity. Once the programming environment is set up, write the code for the smart contract according to the pre-defined rules and conditions. After completing the code, test the smart contract thoroughly to ensure that it works as expected.