Columns display in the order they are added to your smart set.
When creating a new smart set simply add the columns you want to display in the order you would like them to display.  

You can also edit an existing smart set and uncheck the columns then recheck them in the order you would like them to display.  You can utilize the Test Rules button to verify your changes.
​
Be sure to save your smart set after making any changes.
Please note that certain default columns can not be moved or removed from a smart set.