STEP 1: THE START
Get your tomato plant started in the ground and growing OUTSIDE and wait until it is about 12-18" tall. It just needs to be sturdy enough to survive a normal TRANSPLANT without being too big.
STEP 2: THE CONTAINER
Find a 1 or 2 gallon sized plastic container (2 works better) and slice an "X" slit in the bottom of it. You want to create an opening that's about 5" wide so that you can easily slip the tomatoes roots through it from the bottom. DON'T CUT THE BOTTOM OUT OF THE CONTAINER.
STEP 3: THE TRANSPLANT
Dig underneath the tomato plant in your garden as if you were doing a normal transplant, making sure to allow enough room to get the roots as fully as you can. Carefully shake some of the clunky dirt off the roots, enough so that you will be able to insert them through the slit you've made in the bottom of the container.
STEP 4: THE TRANSPLANT - CONTINUED
Place the container upside down and with your one hand, lift the plastic X slit up so that you have enough room to insert the tomatoes ROOTS in the bottom of the container.
What you're going to want to do is seat the tomato deeply through the X slit so that a significant part of the stem (perhaps half of it) is inside the container. NOTE: The tomato will now be upside down inside your container.
STEP 5: THE SEAL
To close and secure the bottom of the container, I just tape it up with some duct tape (of course you made need to cut a tiny bit of plastic at this point so the root has a little breathing room - I recommend a 1-2" circle or square area.
STEP 6: FILL CONTAINER WITH DIRT/COMPOST
Now flip your container so that it is right side up. Continue to hold and stabilize the plant and its roots, so it does not shift around too much as you begin to add your dirt and/or compost to the container.
Carefully add your soil mixture. Add more than you think you need, as it will naturally compress after you water it.
STEP 7: HANG YOUR PLANT
(This is just an example, if this had been an actual tomato, the plant would be hanging down at of the bottom, with the top of the plant nearest the ground.)
STEP 8: WATER
Do this immediately as you would after any transplant. Add additional dirt if needed.
Your plant should take root just fine and you will water like any other tomato in the garden sitting the right way.
STEP 9: WATCH IT GROW!
You'll be amazed how strong the plant will be, it will actually grow back up toward the sun (only to be weighted down later by tomatoes).
Appreciate any questions and once again, sorry I don't have the actual pics with the tomato. That computer crashed with my older photos!