In order to do this glitch you need to do a mission that you don't have any cutscene to go through, including at the completion. An example is the mission called "Map of Occupied Kirkwall". When you have gotten all of the objectives done other than giving the item to the person, you go to the person with your weapon drawn, and while your weapon is still drawn and you are right in front of the person, you repeatedly press the A button. You will get money and exp repeatedly as you press the A button. The exp is limited to a maximum of when all your characters are level 50 however so be careful how you spend the ability and charater points. The gold though you can do as long as you wish, but when you stop pushing the A button past the mission speech you can't do it again with that mission.
To perform this glitch you must first obtain plenty of the Makers Sigh potion. Begin by assigning your available abilities to your 'Tactics' menu then use the Makers Sigh potion which will remove ALL your abilities. Save and reload that save and go into your 'Ability' menu and you will see that the abilities you assigned to your tactics are already unlocked and you will still have ALL your unassigned Ability points. You can either use the Makers Sigh potion again and save then reload or you can assign your points to other abilities and put them in your 'Tactics' menu and repeat the rest of the glitch for more Ability points. This glitch also allows you to unlock certain abilities in classes without having to go through the entire skill tree.