Tomatoes are a warm weather crop; they do not tolerate frost and do not like cold, wet weather either. Wait till the last frost day and give them some protection.
The best place to grow tomatoes in a northern garden is a greenhouse. We grow tomatoes vertically and space them about a foot to a foot and a half apart.
Plants can’t go from indoors right into the ground; it will put way too much stress on them. First get them used to the outdoor or greenhouse environment while still in the pot. Starting with a few hours in the shade and then leaving them for longer and they will get used to their new location. Now they can be transplanted without being killed in the process.
Dig a bigger hole than the plant is – it helps to loosen the soil all around the plant and encourages root growth. Add 1-2 cups manure to the hole before planting (I use sheep manure). The manure will give the plants a boost. If the soil is very dry, like it usually is in a greenhouse, water the hole.
Be sure not to water the plant to much before transplanting so it holds better together. By turning the pot upside down and tapping on the pot, the plant will come right out.
Now place it gently into the hole and fill the hole with loose soil. If the plant is a bit leggy it can be buried deeper just like by transplanting the seedlings. By very leggy plants you can even lay the plant down to be able to bury more of the stem.
Do not press it down too hard but water the plant right away; the water will push the soil down as much as needed. It is good to make a mold around the plant for watering, so that the water can go directly to the plant and not flow away to lower ground.
Give the plants some protection from the sun during the first days, a Frost Blankets works very well. It also gives some protection from wind and cold during the night. I leave the blanket on for at least three days.
To see more: http://northernhomestead.com/transplanting-tomatoes-in-the-ground