This is key. You can use this same recipe on trout too. Pick a fatty fish, so that it stays moist while you smoke it. Spring salmon is a good pick or steelhead. You want to buy it as fresh as you can find it. It shouldn't smell fishy at all and if you are buying it whole the eyes shouldn't be cloudy. Fillet the fish yourself to save cash or buy it ready to go. Make sure it's fully deboned before you start. Keep the skin on and the fillet whole.
A solution of 50/50 water/kosher salt enough to cover the amount of fish you have
A good test is to taste the solution once you've dissolved the salt. However salty it tastes, your fish will be around the same level of saltiness once brined. I add in whatever flavourings I have on hand, ginger beer, lemon rind, keefer lime leaves and cracked pepper was in the last batch. This is the fun part, so experiment and TASTE the brine. Whatever you taste will be what your fish tastes like.
Leave fish in brine solution in the fridge for 24 hours.
Start a small fire with your hardwood of choice. It will burn at varying temperatures depending on how dry it is etc., so you have to check it every 1/2 hour or so and put another small log on here and there. I have a huge olive oil drum that I had made into a smoker with a vent at the top and bottom, so I can control airflow. If you don't have a proper smoker then you can just prop your Bbq's door open enough to let the air in. If you find that the smoke then doesn't come low enough to cover the fish, get some untreated cedar planks, soak them and place the fish on them (higher in the bbq). Basically, you want the fire to stay burning at a very low temperature, creating smoke for a good 4 hours. If you have a smoker... it's way easier.
Remember everyone - A grill isn't a barbecue! A barbecue is a cooking device made for coal or wood, not gas! I'm sure you could try and do this on a grill, but it wouldn't be the same... prove me wrong, I dare ya!
I had to chop down a lovely fig tree in my backyard, so I use figwood for the smoking. You can use any good hardwood like cherry, applewood or alder. I like these better than hickory or mesquite for sure, fruit wood smoke is lighter and sweeter in aroma. I 'warm' smoke my fish. Yes, yes, there's no such thing, ha! Basically, you start the wood fire at one end of your barbecue and place the fish on the other far side of the grill. If you place your hand where the fish will sit, it should be warm, not hot! I usually put it in for around 3-4 hours, but you should check your fish and fire every 1/2 hour as this time will vary. Place your fish skin side down on the well oiled grill as far away from the flames as possible. Check the fish every 1/2 hour until you see a white film coming out of the fish. It's ready at this point. Test the thickest portion of the fish and check that it's just done, not overcooked! It's way better to undercook the fish, that can always be fixed, either by eating it that day (oh my God, it's soooo tasty) or just throwing it back into the smoker to cook longer. Take this off the barbecue and serve some of it warm to your best friends with a good beer on a hot day. The rest put in the fridge for your second best friend to eat later.