Best way to flush it is drain out all the coolant, fill it back up with a water/rad flush mix then go drive around a bit, and drain all that out, and just keep filling it up with water/rad flush till the water comes out clean.
Some people will tell you up take out the black drains, but most the time the bolts round off then you're SOL
As for what coolant to use I run Fleet Charge, you can get it almost anywhere, I got mine from TSC, the coolant is already premixed with the SCA's so all you have to do is mix it 50/50 with distilled water.
Also these engine take 8 gallons of coolant, so you'll need 4 gallons of coolant and 4 gallons of distilled water.