When performing a Riposte or Backstab, enter your inventory. The next part must be done quickly -- so it is recommended that you reorganize your inventory to have your first two slots be your Estus Flask and any boss soul. Select and use a boss soul, then quickly go back into your inventory and select your flask. Select "Use" once it appears. Select "Yes" when the confirmation box appears. This will show the animation of using the boss soul, but will consume a flask charge -- you will also get the souls from the boss soul. Additionally, this can be done with Firekeeper souls, giving you full health and 5 humanity. This can also be performed with an empty flask, allowing you to heal without using a Firekeeper soul.
Dark Souls Remastered