3. Present your case to your boss in a positive manner
Salary is normally related to performance, which allows the company to increase your grade, or promote you, or to award a bonus. Talk with your boss about how you can improve your performance and contribution to the organization, in a way that will enable promotion and grading and pay improvement. Remember, a pay hike would be given in lieu of achieving agreed objectives or standards and an increased contribution to organizational performance. To make this happen you must also know what process exists in your organization to enable this exchange.
Ask for a face to face meeting rather than try to present your case in a letter. Approach it positively and constructively. Ask what flexibilities exist and what the rationale is for setting and increasing pay levels.