Find a surplus alluminum or stainless food serving cart that you might see used in a hospital or in a bakery. They are usualy about 2' wide by 2.5' deep and around 5' tall. They seal up nice and are mounted on nice casters for rolling in and out of your garage. Then buy one of those cheap propane turkey frying kits you see at Albertsons, Lowes or GI Joes and Two 2 quart stainless bowls. If your cart didn't come with racks get some old oven racks from some thrift stores and don't use sheet pans to smoke on . Then most important of all find a dial type thermometer or something else you can check the inside temp of the smoker with that dosen't require opening the door and has a range of 120 to 220 degrees. Now drill or cut a hole in the back near the bottom of your cart that is just big enough to run the propane line into the smoker so it can be conected to the cooker. Next drill a small hole near the upper part of the door to stick your thermometer into. After you brine up your fish (4 parts brown sugar 1 part salt a little corse ground pepper heavly coating your fish in a ziploc bag over night will do) rense and lay the salmon on the oven racks and let dry while you load up the two bowls with whatever kind of wet wood chips you like. (i like apple, alder and cherry for salmon) then cover the bowls with foil and punch a few holes in it. light the cooker and set almost as low as posible. once fish is loaded and a bowl of wood is loaded onto the cooker shut the door and bring the temp up to between 145 to 165 F for around 3 to 5 hours swaping out bowls of wood chips when smoke stops leaking out. For the last hour I like to crank up the heat to around 185 to make sure it is ok for my kids to eat. If you are doing a lot of fish and have more than 3 or 4 racks going at once you may want to pick up a small cabnet fan to circulate air inside the smoker and keep temps even top to bottom also a drip pan can make things a lot easyer to clean up.

There is thousands of ways to smoke fish some hotter, more colder, longer, shorter With fancy wet brines simple dry brines some have to have booze, garlic, apple cider,hot peppers and be coated in brown sugar while they cool. Have fun finding out what you like best and practice PracticE PRACTICE.
Be prepared, your going to need to catch a lot of fish for this...