Sequential deep planting is potting the tomatoes up into larger pots in between seed growth stages and garden planting stages. You may already do this but I think it's worth talking about. Here's how I did it.
I used peat pots. You can get other mediums some of which are made of other biodegradable materials, like cow manure, but these were easy and convenient. Newspapers or cardboard would probably work as well.
Then I took my tomatoes, some of which were 6-8 inches high, and had a short photo op. They like to pose. They are pretty aren't they?
I put the tomato plants right into the empty peat pots. You can put a little dirt in the bottom if you want, either way should be fine. Right now I want to generate as many roots along the stem as possible and shorten the length of the stem. Shorter, more compact plants with more roots grow better than their long and legging counterparts. This is especially true if you are buying transplants from the store. Definitely avoid the leggy ones. (You can trench plant those if that's all the store has.)
After filling them up I added water and put them in our garage greenhouse to help harden them off. In a couple weeks I should be able to plant them outside with nice sized root balls ready to grow. That's when I'll plant them deep, for the second time.