This is from Engine Ice's website:
For any coolant and anytime you are changing coolant, we recommend a simple solution of distilled water and white distilled vinegar, both available at your local grocery store.
1. Mix the water/vinegar at 50/50
2. Fill system with the solution
3. Run engine until warm
4. Let cool
5. Drain
6. Run clear water through to rinse out vinegar.
7. Fill with coolant of choice.