Trim the fat cap to about~1/4 in.  Do a good salt and pepper rub on the entire outside to give it a nice bark.  I smoke it at 225-235 usually and plan on 1 hr 15min a pound (not exact but about what to expect).  Make sure you know where your hot spot is in the Green Mountain and put the point of the brisket towards the hot spot.  I am a fat cap up kind of guy, others may disagree but it doesnt make sense to me to not have fat cap up.  Smoke it until you get to 155-160ish temp in the point and flat.  I texas crutch with either butcher paper (when available) or tin foil.  Foil will usually cook it faster.  I would recommend, especially since it is your first time on the GMG, to USE the texas crutch (wrap it in foil).  It will help you get through the long stall that usually occurs at 170ish and keep some moisture in.  I also spritz mine with apple juice every hour or so prior to wrapping with tin foil.  Once the meat probe is reading 185, I start probing the flat with a toothpick.  It usually will not be done, for me, until that meat probe reads about 194+ but temp is irrelevent when dealing with the doneness of a brisket.  You know its done when the toothpick goes into it like a knife through butter. 

Once it is done *** Dont start cutting it.  Wrap that brisket in a towel (still in the foil) and put it in a cooler for at least 30 minutes.  It will allow it to reabsorb a lot of the moisture.  If you cut in to it right off the grill it will look beautiful but will likely be dry throughout.