I usually don't measure the water precisely, just fill a sanitized glass up with some tap water, so maybe 4-6 oz lukewarm water. Ideally the water should be between 90-100F. Pour the yeast into the glass, cover it with some sanitized foil and let it rest for about 15 minutes or so. Once the yeast are no longer sitting dry on the top of the water, it's okay to give it a gentle stir to break up any clumps.
At this point, it doesn't take much yeast per 12-oz bottle. Just a few drops of dissolved yeast should do the trick. If you have a medicine syringe, lots of them don't have measurements less than 1 ml, so just eyeball maybe 0.5 ml and add it to the bottle. Precise quantity isn't all that critical, you just want to get some active yeast in there. If you are using bigger bottles, just bump up the quantity proportionally.