Pick out a delicious looking corned beef with the LEAST AMOUNT OF FAT POSSIBLE. This is extremely important for the final product. Too much fat, the meat doesn't get dry enough.
After you have picked out your corned beef, boil it with the seasonings for 4 - 5 hours. Seriously. Don't skimp on the time in this step. After boiling, take the meat out of the water and scrap off all the accumulated fat.
Combine the mustard and the brown sugar until it forms a nice thick paste, enough to coat the entire piece of meat. The amount of mustard and brown sugar necessary depend on how much meat you have, so just use your eye to figure it out. Coat the meat and put it in an oven at 400 degrees for 45 minutes.
The finished product should be caramelized on the outside, and the meat should come apart in shreds. Yummy. :) If your meat looks wet on the inside, something is wrong. There's probably still too much fat on the meat - next time, boil it for longer.