1. Create a mapping variable to store the address as a key and whitelisting status as the value. Marking the mapping as a public variable will create a getter function that can be used to verify the whitelist status:

mapping(address => bool) public whitelist;

2. Create a function to add users to the whitelisting contract. This function should be restricted to the owner or admin:

function addToWhitelist(address _investor) external 
    onlyOwner { 
    whitelist[_investor] = true; 
}

3. For easy whitelisting, you can have another function that accepts an array of addresses:

function addMultipleToWhitelist(address[] _investors) 
    external onlyOwner { 
        for (uint256 i = 0; i < _investors.length; i++) { 
            whitelist[_investors[i]] = true; 
        }
}

4. Create a function to remove a user from the whitelist. It can come in handy if an address is added by mistake, or the user is no longer invited to the function:

function removeFromWhitelist(address _beneficiary) external 
    onlyOwner { 
    whitelist[_beneficiary] = false; 
}

5. Create a modifier to prevent non-whitelisted users from accessing the functions:

modifier isWhitelisted(address _investor) { 
    require(whitelist[_investor]); 
    _; 
}

6. Apply the modifier to the token buying function. It will allow only whitelisted users to buy tokens through this crowdsale. The final contract should look like the following example:

contract WhitelistContract {

    mapping(address => bool) public whitelist;

    /**
    * @dev Check if investor is whitelisted.
    */
    modifier isWhitelisted(address _investor) {
        require(whitelist[_investor]);
        _;
    }

    /**
    * @dev Adds an address to whitelist.
    * @param _investor Address to whitelist
    */
    function addToWhitelist(address _investor) external 
        onlyOwner {
        whitelist[_investor] = true;
    }

    /**
    * @dev Adds list of addresses to whitelist.
    * @param _investors Addresses to be added to the whitelist
    */
    function addManyToWhitelist(address[] _investors) external 
        onlyOwner {
        for (uint256 i = 0; i < _investors.length; i++) {
            whitelist[_investors[i]] = true;
        }
    }

    /**
    * @dev Removes single address from whitelist.
    * @param _investor Address to be removed to the whitelist
    */
    function removeFromWhitelist(address _investor) external 
        onlyOwner {
        whitelist[_investor] = false;
    }

    // Add isWhitelisted modifier to token buying function
    function buyTokens(address _investor, uint256 _weiAmount)
        public isWhitelisted(_investor) {
            // ..
    }

}