How to make buttermilk out of regular milk?

Directions

1. 1
   Place buttermilk in clean qt.
2. 2
   glass jar.
3. 3
   Heat milk in microwave until just warm (about 90 degrees).
4. 4
   Add milk to quart jar.
5. 5
   Cover top with a thin cloth and secure with rubberband.
6. 6
   Put in warm place for 24 hours or until it clabbers.
7. 7
   (I put it outside in the shade during the summer and on top of the refrigerator in the winter.)