This first video about balance in a sitting position should be preceded by the videos presented under principles of balance https://celloessentials.com/principle-of-balance/.  After the basic principles of balance in a standing position are understood, then proceed to sitting balance:

Many people benefit from sitting on a cello chair.  The cello chair has a slant from back to front, and is designed to reduce strain in the lower back.  This video demonstrates the principle.