It’s important to take the time to document your code. It will help developers and users to understand the contract and its functionalities. In Cairo, you can add comments with //.

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Best Practices

Since Cairo 1, the community has adopted a Rust-like documentation style.

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Contract Interface

In smart contracts, you will often have a trait that defines the contract’s interface (with #[starknet::interface]). This is the perfect place to include detailed documentation explaining the purpose and functionality of the contract entry points. You can follow this template: 
#[starknet::interface]
trait IContract<TContractState> {
    /// High-level description of the function
    ///
    /// # Arguments
    ///
    /// * `arg_1` - Description of the argument
    /// * `arg_n` - ...
    ///
    /// # Returns
    ///
    /// High-level description of the return value
    fn do_something(ref self: TContractState, arg_1: T_arg_1) -> T_return;
}
Keep in mind that this should not describe the implementation details of the function, but rather the high-level purpose and functionality of the contract from the perspective of a user.

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Implementation Details

When writing the contract logic, you can add comments to describe the technical implementation details of the functions. Avoid over-commenting: Comments should provide additional value and clarity.