Decide what strength you want to make, 5grams=5000 micrograms so adding 1ml= 5000mcg/ml or 50mcg per unit on an insulin syringe, 2ml=2500mcg/ml or 25mcg per unit on a slin pin,etc. The vials are probably 2-3ml so add the water slowly and becarful to note how much you added, preferably an even 2-3ml. Then calculate the dose and write it down.
I would choose to put as much liquid in as I could so there will be fewer mcg per ml and easier, more accurate dosing. Just inject the bac water into the vial. Don't inject forcefully directly into the powder, aim at the side. Roll it around, don't shake, it will dissolve easily. Don't reconstitue them all at once, only one at a time when you are ready to use it. Keep the prepared and unprepared ones in a refrigerator.