Add water gradually by adding in 1/4 - 1/2 of the total amount each time, stirring well after each addition.
Add the egg or substitute.
Cover batter and refrigerate for 1-2 hours (optional. I've, opted not to do this when I am in a hurry. For best nutrition, soak the buckwheat mixture prior to adding the egg).
Ladle batter onto a prepared skillet, greased as necessary.
Cook until galette starts to brown.
Flip galette over and cook on the other side.
Notes
You can soak the flour and liquid overnight as well, and the pancakes will turn out quite a bit thicker. See How and Why to Soak Grains.