1. Log in with the Admin credentials 

2. Go to Settings -> Impersonate User Mapping

3. From the User Dropdown select the user who would be impersonating 

4. From the Unassigned User box select the user to be impersonated

5. Using the double arrow key shift the user to the Assigned Users box

6. Hit the 'Save'

7. The ask the user who you gave access to impersonate another user to login and go to "Settings -> Impersonate User" 

8. On this page the user should see all the people he or she can impersonate. Click on one the "Impersonate User" button next to the user you would like to impersonate and you will be logged in as that user