To use an Action set you need to go into a Work Trigger or Add-In and add an action type of "Run Action Set."




Once that action type is selected you need to specify which Action Set is to be run during this Workflow Trigger's or Add-In's processing.



Now when the above Trigger is activated, all the actions in the Action Set will be executed. If you change the Action Set after this then the Trigger will start executing the new actions. 


If you defer the Action Set then its actions run later. Refer to Deferred Actions.