How do you add a 300a subpanel from a 200 a main panel to install tankless water heater

Biggest instaheat for house I have done, was 100 Amps. I hope you meant 30 Amps, with #10 wire,
If this is the Case. You will need Number 10 wiring, 2 hots and 1 ground. I would get like an indoor disconnect(You see them used at inside Air Conditioners)A 2 pole breaker in the Electrical Panel, You don't need a breaker in th disconnect.

Strat... I've seen some huge power hogs for these things. A 30A heater would be a VERY small heater.

Arubbo... most homes I've seen with tankless heaters large enough to properly serve the home have 300A to 400A service. You are going to need to upgrade your main panel and service to your home.

Cobra, Are you thinking he has a 300 Amp Instaheat? He could, I'm thinking Typo?

I'm not sure what he meant to type Strat. But for example, I saw a Stiebel model recently that was a 150A unit that required three separate 60A breakers. That was a whole house unit. If it's just a 30A unit, it's a small one for the kitchen only, etc.