Welcome to the forum Ian!
Take that butt and salt it all over using kosher. Salt the entire roast. Let that sit for 15 - 25 minutes. Then rinse it all off with cold tap water.
Pat it dry. Then season it with your favourite rub. Season it good. Wrap it in Saran wrap and refrigerate it over night. Next day, open that Saran and season it again. All over.
Fire up one WEBER charcoal chimney with Kingsford. While that's going, 3 quarter fill your coal chamber with unlit Kingsford.
Choose your favourite smoke wood and have at hand. When the chimney has red hot coals at the top, dump it over the unlit coal in the chamber of the WSM and even out.
Now get that butt out of the house and on the top rack of the WSM. Get your temperture probe in the roast away from the bone of the butt.
Add your wood to the hot coals and assemble your WSM. Open ALL vents until the dome temp hits 235-250'F then close the bottom three half way.
Find a cold beer/refreshing cocktail and smoke on!
Figure this cook about 1 and a half to 2 hours per pound. I remove my butts at an internal temp of 195-205'F ... in that range. Then I tightly foil it, set it in a dry cooler for at least one hour then pull it using
bear paws.