You have to be definitely be near the stove always stirring the mixture otherwise it easily forms lumps.
I have used samai rice for this usually rice noi/broken or even boiled rice is used.
Buttermilk that is to be mixed can be watery no worry it will be more tasty.
Soaking ragi is the usual method done here and it tastes very good.
I have added small onions and chilli for spiciness and crunch.
If you want to avoid chillies can leave and it can be accompanied with spicy pickle or even spicy curry/kulambu.
Use heavy bottomed vessel and cool the mixture before mixing with buttermilk.
You can refrigerate this ragi koozh without butter milk for more than a week and u can take out when needed mix with buttermilk and use.