You can refer to Seema Sanghvi's book on competency mapping. The basic process of Competency Mapping starts from:
1. Assessing the organizational hierarchy of an organization or department.
2. Determining the level you want to map—senior, middle, or lower level.
3. Creating a Competency dictionary, e.g., if you are looking to map the Manager HR position, then you should first know what competencies the person in that role must have to operate at that level.
4. Applying research tools like questionnaires, surveys, etc.
5. Conducting a 360-degree feedback session from employee-employee, employer-employee.
6. After gathering all the results, map the gap between the actual and expected competencies of employees.
Hope this will help. It's better to use rating scales methods in Competency Mapping.