Loins work great on a charcoal grill with indirect heating. All you need is some charcoal, wood chunks, seasoning, and a meat thermometer.
Whether you season your meat the night before, or just before you put it on the grill, is your choice. Use your favorite seasoning.
When you are ready to start hot smoking your pork loin, open all the vents on your charcoal grill. Put but about half a chimney of cold charcoal over to one side of your grill. Put 3 fist sized wood chunks (fruit wood works nicely with pork) on top of the cold charcoal, then spread another half chimney of glowing hot charcoal over the top of your cold pile of charcoal and wood.
With the top off the grill, let that all heat together for about five minutes.
Put the top on with the top vent opposite your fire. Let your fire run for another 15 or 20 minutes, until you see the smoke start to diminish a little. Then add your meat on the grill opposite the fire.
Your target temp of your meat is going to be 140F internal temperature on the meat thermometer. Depending on the size of your loin and the heat of your chamber, the meat can take 1-2 hours to reach that internal temp.
Once the meat is at 140F, remove the meat, cover, and let it rest for 10-15 minutes.
Slice and serve!