I'm reposting a procedure I've used. Best to have a new clamp on hand as they have been known to break.

Power flushing is a simple DIY job. I use synthetic Mobil-1 ATF or Amsoil. You will need up to 15 quarts.
Disconnect the upper cooling hose. Open the metal clamp carefully and wiggle, wiggle, pull, pull. Cover the hose end with a baggie.
Clamp 6 ft of clear plastic 1/2 inch ID hose (hardware store item) over the hose end and prepare to drain into gallon bottles or a calibrated bucket, so that you know when you have removed about 12 quarts. Idle the engine to pump out fluid. Shut off immediately when bubbles appear in the clear hose. Add two quarts down the dipstick hole using a tranny filler funnel and hose from auto supply or chain store. Repeat the pump/fill cycle. Somewhere in the middle of this you may run the shift thru D N R while idling to assist the clean out. Once you've removed 12 quarts, the fluid should start running clean shortly thereafter, at which point reinsert the cooling hose into the radiator. Top off the level per the owner's manual.