Remove the plant from the nursery pot and place the tree in the container. Add prepared soil mix. The graft should be at least three inches above the soil level. Water when necessary. Don't let the tree dry out or be too wet.
Good drainage is essential.
Fish emulsion or other organic fertilizers both liquid and dry are excellent. Fertilize only when the plant is growing new leaves. Nitrogen is the main requirement, a diluted amount of fertilizer every time you water works well. Best of luck. In the past, heavy rainfalls either before or during the planting season has done a number on my tomato plants, whether they grown from seeds or purchased as plants.