What is the best way to shorten legs on a table and chairs?
Put the table/chair on a very flat and very horizontal surface. Ensure and double check with a levelgauge.
shim the table/chair so that the top surface of the table/chair is level using the level gauge
Take a pencil and tape it on a small block that is about the height you want the legs to shorten. Ensure that the block has parallel bottom and top planes.
With the pencil on the block, scribe the legs of the table/chair and make marks all around. 
Cut precisely and carefully with a proper saw. Which is probably the hard part. If you start on a corner of a (square) table leg, make sure you cut on the cutting line on two faces.


in short:

Watch this video by Chris Schwarz: level the feet of a chair or sawbench