1. Make sure to put enough soil in your planter. It should be almost to the top (or bottom depending on how your planter is currently) of the planter save for enough room for your
fertilizer in step 2.
2. Add extra compost to the top of your planter. About 3 inches (6 to 8 cm) of compost to the top. You can do the same with a denser fertilizer like any standard, garden variety fruit fertilizer.
3. Plant your plant! This works best with a transplanted tomato plant that way it holds on to the soil upside down. You can do it with a seed, but it is much more difficult to grow a tomato from the seed upside down.
4. Water the top of your planter. If you see water coming out of the bottom you're over-watering it.
5. Repeat! Make sure your tomato gets plenty of sunlight on its green leaves.
6. Do not sucker (prune the new growths at the base of each branch) the tomato plant. You'll end up getting a LOT less tomatoes if you do this
Credit goes to Upside Down Tomato Growing Instructions