With Outlook open:
Click on the File tab.
Click Account Settings, and then click Delegate Access.
Click Add.
Type the name of the person you want as a delegate. The delegate must be a person in the Global Address List.
Click Add, then click OK.
In the Delegate Permissions dialog box, choose what type of access you want the Delegate to have.
Click OK.
Here is video that will also show you step by step.
https://youtu.be/wypG0Ggf4mU