Transplant your angel trumpet seedlings into individual two-inch clay pots when they're large enough to be handled. They should have at least one set of true leaves, and two sets would be preferable. Gently water them enough to evenly moisten the planting medium, but not so much that it's soggy or wet.
Cover the pot with a clear plastic bag to retain necessary humidity for the seedling. Poke several small holes in it to encourage air circulation, and secure it into place with a rubber band. Place the mini-greenhouse in a warm, brightly lit spot out of direct sun. The top of a refrigerator or above a hot water heater are excellent locations.
Continue to keep the potting medium moist. Remove the plastic bag whenever humidity levels rise significantly. In most homes, this would be when the weather warms up to the point that you're no longer using artificial means to heat your environment. Alternately, if you're using a humidifier in your home, the mini-greenhouse that the bag creates probably isn't necessary.
Begin acclimating the angel trumpet seedlings to the outdoors after all danger of frost has passed. Set them outside in a bright, lightly shaded spot early in the morning for a couple of hours each day. Bring them in before the sun gets high. The following week, they can stay out until noon. Put them in full sun in the mornings of the third week, but move them back into light shade during the afternoons. They can now stay outside all night.
Cut all of the leaves off of the now established young angel trumpet plants. Leave only the upper bud. These plants always wilt when being transplanted, but removing leaves will minimize the damage. Set them in the sunny spot you've chosen for their final home, but leave them potted for a week to monitor their progress. Keep a close eye on their watering needs, and make sure the medium remains moist.
Transplant the young angel trumpets in a well-draining location in full or lightly dappled sunlight where they'll receive some shading from the hottest afternoon sun conditions. Brugmansias love rich soil with a neutral pH. If possible, locate the young plants near a building or larger plantings to shield them from strong winds. Remember that these plants can grow into large shrubs, so give them plenty of room to spread. Position them at least two to four feet apart, and make sure that you plant the angel trumpets at the same level that they occupied in their growing containers.
Water slowly and deeply to thoroughly dampen the plant's root system throughout the blooming season. Allow the soil to dry out slightly before watering again. Apply a layer of mulch to help retain moisture and discourage weeds.
Feed slow-release 12-16-12 fertilizer every two to three months throughout the growing season, if it will make you feel better. This really isn't necessary because the angel trumpets thrive and grow rapidly without it.
Add another heavy layer of mulch to outdoor angel trumpets before the first frost is expected. This will help them to survive dips in temperature slightly below 32 degrees, as long as the roots are adequately protected.
Bring potted Brugmansias indoors when cold weather nears. Even if they survive a frost, they're likely to suffer damage from it. Set the plant in a warm, brightly lit spot out of direct sun and feed it once monthly. Water enough to evenly moisten the soil, and let it dry out slightly before watering again. Angel trumpets can be gradually re-acclimated to the outdoors after all threat of frost has passed in the following spring. If at all possible, large clay containers are best for angel trumpets because they afford the best possible drainage.