1. Create a User
2. Create an Employee (Makes sure it has a Job Title)
3. Assign this employee to User created in # 1.
4. Make sure that the Job/Role is in approval groups created in purchasing.
5. Depending on the position based or employee-supervisor based approval method, you should be able to approve the Req's.
6. If its employee-Supervisor based then if you are a Supervisor, you can approve a Req if the Req is created by your sub-ordinate.
7. If its position based, then you should be able to approve if the req is created by a position reporting to you or equivalent.
You can define the user as approver using position hierarchy in purchasing module.
you have to assign the approval group to user's position for approval of PR.