Imploring your Patron's aid requires you to use your action. Describe the assistance you seek, and roll percentile dice. If you roll a number equal to or lower than your Warlock level + your charisma modifier, your patron intervenes. The DM chooses the nature of the intervention; the effect of any Warlock spell or Warlock domain spell would be appropriate.

As this is an extension of your pact, you can bargain with your patron to increase your chances. Before your roll, you can choose to make a sacrifice in order to appeal to your patron and change the target number. You can suggest a sacrifice, have the DM make you an offer, or you can agree to an undisclosed consequence of the DM's choosing.

Once a bargain has been struck, if you roll a number equal to or lower than double your Warlock level + your charisma modifier.

The Bargain occurs regardless of the outcome of the roll. You make the deal, pay the price, and get the advantage. 

If your Patron intervenes, you can’t use this feature again for 7 days. Otherwise, you can use it again after you finish a long rest.