I have the best luck filling it with the motor off, bleeders open. Fill until it flows from the bleeders. Close everything up, start the motor and let it warm to operating temp. Then shut it off and wait about ten minutes or so, then carefully remove the cap and open the bleeders again. Fill as before, close when coolant runs from the bleeders. Road test.
You HAVE to road test, with air they can sit there and idle cool all day, then suddenly spike as soon as you start driving it...I've even seen some sit in the shop running at 2000 rpm for 20 minutes running cool, and overheat within a few blocks when driven...bizzarre...
for some reason filling while running seems to complicate the process for me...
I really like the way they did the 2.5, where you pull the 'stat out and fill it from there...great idea...wish they'd done others that way.