Follow these steps to create mappings in solidity:

1. Mappings are created with key-value pairs. A mapping can be compared with a hash table or dictionary. The following example creates a mapping of an address and a user-defined type, and another mapping with an address and an unsigned integer. These are used for storing employee data:

contract test {

    struct Person {
        uint id;
        string name;
    }
    
    mapping(address => Person) employees;
    
    mapping(address => uint) balances;
    
    function insert(address _employee, uint _id, 
        string _name, uint _balance) public {
        employees[_employee] = Person({
            id: _id,
            name: _name
        });
        balances[_employee] = _balance;
    }
}

2. Mappings can be nested to create a more complex data structure:

contract test {
    
    mapping(address => mapping(uint => bytes32)) dataStore;
    
    function insert(uint _id, bytes32 _value) public {
        dataStore[msg.sender][_id] = _value;
    }
}

3. In a mapping, the key can be anything except a mapping, a dynamically sized array, a contract, an enum, or a struct. The value can be of any type.

4. The key data is not actually stored in the mapping, as only its keccak256 hash is used to look up the value. Because of this, mappings do not have a length associated with them.

5. Mappings are not iterable, but we can design a data structure to allow such operations.

Declaring an array or mapping as a public variable generates a getter method for it. In a mapping, the key will become a parameter for the getter and it will return the value. For an array, the index will become the parameter.