Here is a simple first time process.
1 parts kosher salt
3 parts medium grind black pepper
.5 parts chile powder
Rub entire butt with crushed garlic, then apply rub liberally, let sit for an hour, put on smoker. This should give you a good baseline to start from. You can't go wrong with buying a commercial rub, I always like Pappy's as a widely commercial rub.
Whip It Off, Chambers!
"perhaps...but then again...maybe not..."