You must be holding your weapon 2 hand or have a weapon/shield in both hands, if you have a free hand apparently this dupe glitch doesn't work. Try holding a different weapon as well, some don't work so well. There have been some reports that this doesn't work as well on characters that are female. This works on the latest patch on console and PC.
You attack (or roll/backstep/parry), press use item, then during your animation press use soul, then open your start menu, and select and use the estus from your inventory after your animation has finished.
You can infinitely use boss souls, homeward bones or fire keeper souls. A fairly simple trick, make sure to use only for good and not for evil.