How can I set my listings to automatically relist when they end?

First, you need to create auto relist rule. To do that, see How to create Auto Relist Rule?

Then, you need to apply the rule to one/more listings.

Follow these steps:

Select one / more listings from the Grid

Do right click inside the Grid. You will see many options displayed.

Click on the option "Set Relist Rule". A dialog box will pop up asking you to select the rule you wish to apply to the listings you selected.