What is the proper way to drain a boiler's expansion tank?
Turn off the valve that auto-fills the system from your cold water line
If you have one, open a valve or bleeder on the top of your expansion tank.  If you can't do this, it will still work, the water will just gurgle and drain slowly
Open a drain valve and drain a suitable amount of water
Close drain valve and re-open valve you closed in step 1


Amount of water to leave in tank:  If the system is cold, I would leave the tank close to empty, that maximizes expansion space.  If it's warm or hot, I'd go for about halfway.  You can tell where the water is by feeling the temperature of various spots on the tank and the pipe that feeds the tank.