Look at the post in the link below - I posted step by step on what I've done to get a great brisket. - this one was a 12.4 lb packer - so pretty close to yours.
I always do packers - I love the great differences in the two muscles.
I estimate my cooking time at 1.6 hrs. per pound plus 2 hours for a temp around 225F - but the internal temp is what tells you when to truly pull it out.
I just dry rub with a little plain old yellow mustard to help the rub stick.
I cook fat side up - the fat rendering drips through the meat adding great flavors.
Look at the trimming I how in the thread I posted.
be patient - and let it get to temp - often you will get a stall where it sits for a long time and the temp does not rise much - hang in there - it will get past the stall.
Have fun and enjoy!