There are many theories out there when it comes to watering, one of which is trying to water plants with milk. While this is, of course, better than pouring soda and beer over your soil, "watering" your plants with milk straight from the carton won't benefit your plants. The key to using milk when watering is to make a dilute concentration, so you don't encourage disease or garden pests to come to your garden.
Save the milk jug as soon as you pour off the last of the milk. There will still be a small amount of milk left in the bottom jug and along the sides.
Fill the jug with water up to the top, swirling the water along the sides of the jug as you fill it.
Use this milk water to water houseplants, outdoor container plants, or small garden beds. This water should be used in place of regular watering, not in addition to your normal watering routine.
Place the milk water in the fridge if you aren't going to be able to use it immediately to keep it from souring. Just be sure to allow the jug to sit out and warm back up for an hour or two before you use it.