I just flushed my system the other day. I parked it up on ramps and ran it up to temp with the cap off. it puked out a bunch of air and a little coolant. I had to rev it a little to kick out some of the air thats when the coolant puked out so make sure you have a drain pan under it .
You have to get the front end as high as you can. I parked on the uphill grade of my driveway, with the front tires up on ramps. Leave the cap off and be prepared to add coolant to the rad as it burps out the air.