Fill Jar no more than 1/2 full with heavy cream, add salt if desired.
Screw cap on tight and shake. After a few minutes will turn to a whipped cream. Keep shaking and all of a sudden it will loosen up again and you will hear the butter hitting the jar and hear the separated buttermilk liquid.
Stop shaking and drain (I like to do this into a cup for drinking)
Spoon out butter in to bowl and use now or store or put into a mold(it gets hard when cold).
tip- I found if the cream gets so whipped it is hard to shake from the sides a tablespoon of warm water added will loosen it up and speed up shaking time.