Here is a video tutorial about the creation of brushes (stamps/stencils) in Krita. I often create brushes 'on the fly' when I have to paint something repetitive (small rocks, bubbles, leaves, herbs, etc... ). It's easy and quick to create this type of brushes with the right method. With this video, I hope I'll share this knowledge.
[edit 2020] WARNING: Since 2016 (release of this video), Krita developers changed the dialog: the checkbox "[ ] Create mask from color" in the Stamp panel is now checked by default and you'll have to uncheck it manually to preserve the color of the stamp. My bad for telling in the video to keep all the default, I'll learn next time I do a video to explain the option in case they change.