Hi Nancy - my son, Brady, is 3 months old, and I dissolve it in breast milk. To be honest, I use sodium chloride prescribed by Brady's doctor. I was never able to get him to take the actual salt dissolved in breast milk - he just gagged and wouldn't swallow it. I tried it myself once - I put salt in regular milk and tasted it! It tastes horrible, no matter how much milk you use to dissolve it. So, I definitely recommend dissolving it in as much breast milk as you can.
Another thing Brady's doctor suggested was not to give him the entire amount of salt (1/8 teaspoon) in one feeding, to spread it out throughout the day, so that you're using less salt with each feeding. You might want to try that depending on Maggie's feeding schedule. I breastfeed, but I pump and then fortify it with formula because Brady was born very small, and we're trying to get his weight up. So, he's on a pretty strict schedule with his eating.
Good luck!