Flushing engine coolant on a 2001 Triumph Sprint RS, which had a neglected coolant system . (Fluid didn't get changed for over 7 years and full of rust)
Things to note:
Make sure you use distilled water when possible. Avoid the temptation of using tap water. Make sure to flush the radiator with distilled water or engine coolant.
After 3-4 month, drain the coolant again and flush with new coolant, you'll find there will still be some rust in the coolant. This will get better as you continue to maintain a regular coolant flush interval.
Since then I have switched to Dex-Cool (red) coolant (after completely flushing the regular coolant (green) out of the system with distilled water, which I found works a little bit better.