What would you recommend for someone's first low-and-slow cook?

Someone has a free Saturday, a grill or smoker, and wants to try their first low-and-slow cook. What would you put on the grate? Tell us what cut you'd choose, why it's a good starting point, and the one thing you'd explain before they get going. If you've tried it on a pellet grill, an offset, or a regular grill with a smoke tray, share what worked for that setup. If you're planning your first cook, jump in with your questions. You don't have to know the terminology to ask.