What does everybody do when out with a formula fed baby?

So how does the whole water thing work?

If I put boiling water in a bottle and kept the formula separate and then added the formula when it was feeding time would that be ok?

How long can you keep the water for?

What does everyone do when you’re out and about??

I have ready made stuff but I don’t want to use that all the time

***EDIT****

I’m not worried about heating the bottle up, I mean how long will the water be ok for in my bag? How long is it sterile and ok to use for? As in I know you can only keep a bottle of formula for one hour