Once the danger of frost has past and you have adequately hardened your plants, you are ready to plant your tomatoes. The soil should be well tilled and free of weeds. I use a post-hole digger to dig a hole deep enough that I could put the entire plant in it. Then I will add several inches of water to the hole. Allow that to seep in for a few minutes and then fill the hole back in with 6 inches of soil. This is particularly important where I live where the dirt has a lot of clay in it, but it will help the roots spread into any soil more efficiently regardless of your soil type. I then carefully remove my plant from its container, trying to move the plant and the soil together and place that into the hole. You should have 6 inches or less of the plant above ground. You should remove any leaves/branches below the soil line and then fill the hole.
I like to add a couple of inches of mulch around the plant and then water it heavily. The mulch will help keep the soil from drying out so much, and it will suppress some of the weeds. While watering in, I like to add some water-soluble plant starter with some of the water to give the plants an extra boost. My personal favorite is Southern Ag Plant Starter. I believe it’s a good practice to plant your tomatoes late in the day, so the sun isn’t beating down on them immediately after planting. On the following day, I will check on the plants to make sure they have adequate water and to make sure they are not getting too much sun. It’s helpful to have some row covers you can place over your new plants if it’s a very sunny day or even if it happens to be extremely windy. If you can be protective of them for the first few days, they should be fine after that.