Basic guide:

www.ellenskitchen.com/forum/messages/8118.html

Here is a reliable selection of tested baked or oven roasted corned beef recipes:

www.cooks.com/rec/search/0,1-0,baked_corned_beef_brisket,FF.html

Electric raosters:

www.chefsline.com/blog/quick-tips/making-corned-beef-in-a-roaster/

Roaster slow cook is a great method for corn beef, double the cooking time and cook at 250. For three pieces, figure the cooking time on the smallest and add 1/2 hour. Keep the lid on- peeking lowers the temp enough that it increases the cooking time.

Corned beef is best when cooking slowly and with some moisture, more or less braising. Limit the number of times you open your roaster because heat will quickly escape. In an electric roaster, cook the corned beef brisket at 350 degrees; a large roast (6-7 pounds) will take about 5 hours. The cook time for the brisket will depend on your roaster and how often you open the door/lid.

The corned beef is finished when it reaches an internal temperature, measured with a meat thermometer, of at least 145 degrees minimum but it will taste best when 160 degrees and fork-tender.

Preheat roaster oven for 15-20 minutes before adding the meat.

Braising Liquid
Join foil strips, using a butcher wrap to seal the joins, make a piece that will line the whole roaster and wrap completely around the roast and future vegetables.
Place your brisket, fat side up, on overlapping sheets of aluminum foil. Place in roaster pan, then add enough liquid to cover 3/4-1 inch of the way up the roast, about 2 cups. For your liquid, use water, apple cider, apple juice, water with cider vinegar, wine, etc.; it really depends on what you like. Add some orange peels (white pith removed), whole cloves, and 2-3 whole bay leaves, or the spice packet that comes with the roast. Wrap roast by folding top edges together (so you can peak inside to see the roast easily).

After 4 hours when it’s around 140 degrees, drain most of the liquid into a pan- a turkey baster works well for this. Take out the meat, using forks or gloves, and set aside. Place small or quartered potatoes, large chunks of carrots, turnips or other root vegetables, onions if desired on the aluminum foil. Put the corned beef on top of the vegetables, fold the foil back over, and return all back to oven for about 40 minutes. Layer cabbage pieces or wedges all over the top, cover, cook 20-30 minutes until cabbage is tender.

If baking:
Here is the basic baked info:

4-6 pound pieces: Soak in fresh water to desalt, 1/2 hour or longer. Use heavy foil, place beef in the middle, add 1/2 cup fresh water, few slices onion, celery, carrot. Seal with butchers fold, so package is water tight. Bake in a slow oven (300 degrees F.) 4 hours. Cool slightly, unwrap and put in shallow pan. Spread with brown sugar mixed with mustard. Bake in moderate oven (350 degrees F.) 20 minutes or until glazed.