Happy Friday everyone!!
I know this picture is kinda sad but I still think he's super cute, even when he's whining.
Adjust the size of the brush to what you like. For this image I did 20 pixels. (Shortcut: [ to make the brush smaller, ] to make the brush larger)
Change the Hardness to 0%. This makes your brush soft so you don't have any hard lines making it obvious that you edited under the eyes.