use a 2-1 brown sugar to kosher salt ratio for thin fillets. thick fillets (1/2" or greater) can take a 1-1 sugar salt ratio. add to the dry brine whatever you like. i like to add dill, cayenne, garlic powder, smoked paprika, and chipotle powder. sift it until well blended.
use a couple layers of food service wrap in alternating directions.
layer your fillets with dry brine.
then wrap tightly, and then wrap in food service foil.
put in fridge, turning twice a day for two days. after 48 give or take, rinse the fillets, pat 'em dry and place on wire racks in front of a fan for two or three hours. this will develop the pellicle.
don't skip this step or you'll have mushy smoked fish.
this is what your fish should look like prior to smoking. dry, but moist and shiny.
smoke your fish with indirect heat with hickory, oak or pecan at less than 140 degrees. a couple hours should be plenty, depending on thickness.
and don't forget to have clean surfaces, clean hands and clean utinsils AT EVERY STEP in this process. although this isn't technically cold smoking, it's close enough. and the fish is at dangerous temps for long times. i've never had anyone complaining of gastro problems in the many steelhead/salmon smokes because i'm a clean freak when it comes to handling and preparing the fish. you should be a clean freak, too.
special thanks to alton brown and @ribguy.