you have a couple choices for brine, wet or dry
I prefer the dry brine. I also like to freeze then thaw the fish, it will lose some moisture in the process and have a better texture in the end
how i do it:
1 part kosher salt, 3 parts brown sugar (use dark for a better color)
that's the basics, i typically add some garlic and onion powder, and whatever else fixes my fancy, cajun, lemon pepper, be creative.
depending upon how big you filets are i typically cut mine in half, then start layering the fish in a large straight sided pan (roaster works good)
1st layer skin side down add a big 'ole handful of birne, rub it on the fish coating it well, then take another piece again rubbing it good,place it on top of the first layer flesh to flesh, next layer rub again, layer skin to skin and so on till you are out of fish. there should be a lot of rub, and it will "melt" into a goo
cover with plastic wrap, in the fridge overnight, i like to do this around noonish, then before i go to bed, switch the layers around, top to the bottom and repeat layering method
next day AM rinse fillets, pat dry, the fish will have change in consistency, little tougher kinda rubbery is bad way to describe it, place on greased smoking racks and put on counter to dry them with a fan, i suppose you might be able to use your smoker for this, just rack them and put and fan at the door with the door open.put a pan on the batoom to catch and mess, about 4-6 hrs later, there should be a nice sticky coating on the fish., this is key.
now for the smokin, start low and slowly bring up the temp, something like 150 and finish around 200, i can't tell you how long this is going to take as it depends upon the thickness of the filets, i don't use a thermometer in the fish, but i have a good eyes for it, i typically can tell when its done by pushing down on a filet, it will "spring back" hopefully somebody here can offer a temp when to take the fish off. for me someplace between 3-6 hours is typical
now if you are doing a whole fish, a wet brine is the way to go, but thats not my normal deal so ...i'll bow out and let someone else chime in.
a word on wood, i like apple and cherry, but alder works really good too
i can never not eat a loin as it comes off the smoker, not much better than right off the smoker fish