Step 1 - Press the windows icon at the bottom left and then select search symbol at the top right as shown below


Step 2 - type mmc into the window and launch the app that shows up under results as shown below:

Step 3 - Navigate to Local Users and Groups (local) then to Groups then to Administrators on the right side as shown below and double click to open the Administrators window.


Step 4 - Press add button as shown and then type in username of the person you want to be admin for that particular computer as shown in the second screenshot.