I usually cook 3-4 8-10 pound shoulders at a time, on my 18, you'll be fine. Trim as much fat off as you can, (All it does is make a mess in your water pan if you leave it, and there is plenty of fat in the meat to render and add flavor). Lather the meat in what ever binder you want... I've used mustard, olive oil and italian dressing, and have not noticed any difference, all it is doing is providing a binder for the dry rub to stick to. Then generously coat the entire piece of meat with what ever rub you choose. There are tons of choices store bought or home made. I have done this the night before, and also a few minutes before. I really cant tell a difference in the end result.
Set your coals up with the minion method or su's donut. Basically use something similar in size to a coffee can, ..set that down in the middle of the coal rack. Pour charcoal all around the coffee can, fill it up as much as you can fit in there. Add about 3-5 decent chunks of wood....hickory, oak, mesquite, play around you can taste the difference . Unlike chicken and poultry, A pork shoulder can take alot of smoke and not come out bitter or acrid. Remove the coffee can from the middle of the coals, and fill that void with about a 1/4-1/3 of a chimney of lit coals.
Set your smoker up, fill the water pan just about full of water. Get your thermometer probe in there, as the lid is usually off. Wait till the smoke changes from white to light blue and your pit temp settles down. I usually leave my vents open about the width of a pencil and that is perfect. Your pit temp ideally is 225-250, but its not science and it will fluctuate for all sorts of reasons, especially if its very windy. When my WSM was new, it took a good 2-3 cooks to get the feel for it and find the sweet spot.
I suggest spraying the rack w/ some non stick spray it makes clean up easier but its up to you. Put your butt on there, stick the probe in, close the lid and leave it.
At first your internal temps are gonna climb fast. Dont be fooled, around 165-175 you will enter "the stall" and it will take a while (many hours) to come out of it. There is all sorts of articles and stuff on what the stall is, and why, some boring and some interesting.
Regardless, you have 2 options now. ride the stall out and wait, or wrap. If you ride it out, you are just letting the meat do its thing and eventually it will start climbing in temp again. Or you can wrap the shoulder in foil or butcher paper. Usually depending on the time, I will let it sit in the stall for a while and around 4-5 hours in, if it has not moved, I will wrap it with foil.
If you use a full load of Kingsford Blue Bag, you can figure on getting about 11-12 hours out of one load before you start seeing your temps drop. Once they do, I light a 1/4 chimney and remove the center. I pour on a little bit more coal, and then dump the lit chimney on there. If I catch it before its too late, sometimes I skip lighting another chimney, but usually my timings off or there just doesn't seem to be enough coals left to light a new batch.
I start probing my shoulder around 202. I Like to pull it off at 203. I have let it stay on to 205 or more and it was too dry. Less then 203 I found it harder to pull the pork. After, I remove it from the wsm, it is either already wrapped, or I will wrap it in foil, then place it in a cooler, and lay a towel over the meat in the cooler. Let it rest in the cooler with the lid closed for 2 hours or more.
Open it up, and start pulling pork. ENJOY!!
Remember its not exact science. I have done shoulders, where my pit has spiked over 300, or dropped down below 200, when I fell asleep and forgot to set the alarm. All of the cooks came out awesome in the end. Don't sweat it and enjoy!!!
If you have a lot of meat left over, there are some good brunswick stew recipes out there.