How do I feed my 6 months old plain water? After every milk feeding? He is fully on formula milk. How many ml can I give?

There's no real need of feeding water because formula milk itself is about 60-70% of water already :) If you really want to, just a few teaspoons will do!

for a start you can offer 1-2 teaspoon manually. You can also put about 10-20ml of water into his bottler and offer him to suckle.

just give a few spoonful will do. at this age no need to really feed water.