It must be refrigerated. It also must be mixed slowly and swirled, NOT Shaken. Don't squirt the water directly on to powder, let it drip down the side.
As to how much bac water to add, you need to decide on your protocol, ie how much you're planning to run and how much volume you want to inject. If you use 5ml of liquid, for example, then you'll have a concentration of 5000 iu in 5ml or 1000 iu/ml. If you use 2.5ml then you'll have 2000 iu/ml. You get the idea.
If you're planning to run 500 iu ED then I'd go with the 5ml since that'll leave you with 0.5ml to inject. If you're going to inject 1000 iu then I'd go with 2.5 since that'll leave you with 0.5ml to inject. I like shooting 0.5ml