When you're seedlings are big enough to handle pot on into 3-4 inch pots or recycled containers (making sure you put drainage holes in them). Water little and often, keeping them out of frost, wind and draughts.
When plants are 6-8 inches pot on into final pot, or grow bag or in a sunny boarder. Feed your tomatoes once a week, 'Tomorite' works nicely and supermarkets stock this. With tumbler tomatoes leave any side shoots on and plant in patio pots or hanging baskets. With cordon/indeterminate plants such as 'Money Maker' or 'Gardeners Delight' remove all side shoots that appear in the joints of the leaves. We're getting a cold snap next week so postpone planting out until it's warmed up again or your plants can die or go into a state of shock. Have fun.
When you're seedlings are between 6-8 inches be brave and nip the tops out just above a set of leaves. This will encourage them to produce side shoots and have more flowers. Before planting out after the last cold snap, acclimatise them by putting them outdoors during the day and bringing them in at night for a week before planting out in their final position in a bright, sunny spot up a support frame of your making. Feed once a week, tie new growth gently to the frame and dead head regularly to encourage continuous flowering. As summer comes to an end you can stop dead heading and allow pods to develop so as to collect seed for next year.