Wash the milk jug thoroughly. Discard the cap.
Cut off the bottom of the milk jug with the knife and discard. You now have a somewhat funnel-shaped container. Keep the container inverted, with the neck facing downward.
Pierce four holes equidistant around the widest part of the funnel and cut four lengths of wire about 2 feet long.
Pick up one wire and thread it through a hole about 5 or 6 inches. Bend the wire up and twist it around itself. Repeat for the other three holes.
Gather the long loose ends of the wires together as one and then bend them over about 6 inches from their ends. Twist these together, leaving a loop for hanging.
Pick up the tomato seedling plug and turn it upside down in your hand. Working from the wide, open end of your container, reach inside and ease the tomato seedling through the neck of the milk jug. The seedling should be hanging out the bottom of the funnel with the soil trapped by the neck.
Add more soil until it is within an inch or two of the wide end. Hang your plant on a secure overhead hook on a porch or other structure, or hang from the limb of tree.
Wash the milk jug.
Cut the top of the milk jug away and recycle the handle part you removed. Decide how deep a planter you want and cut the jug to that depth. Permanent tomato plants will need more depth than seeds you are growing for transplanting.
Make a few drainage holes in the bottom of the milk jug.
Add your soil and then plant your tomato starts or seeds.
Tend your tomato plant as normal.