You really shouldn't be injecting anything without basic knowledge of the compound. 2mins of research would tell you how.
But seeing as IPT-141 isn't really that dangerous as far as I'm aware... the amount of water is up to you. The more water you put in the weaker the concentration.
E.g: 10mg powder + 10ml of water = 1mg per ml. So if you wanted to inject 1mg, you'd inject 1ml of the solution.
or 10mg powder + 5ml of water = 2mg/ml. So if you wanted 1mg, you'd inject 0.5ml.
Concentration is up to you. If your going to inject subq you should aim for a stronger concentration so you can keep the volume below 0.6cc per shot.