duckwing
You can place the smoker box on the flame spreader bars, "flame guard", flavorizer bars or what ever your grill mfg. calls ‘em just below the actual grill grid. You can also put it on top of the grill grid if you have space. There’s a lot of differing opinion on whether to soak the wood chips or not, how high the temp. should be etc. Both wet and dry chips will work – depends on where you put the box and the burner temp. I have one of the cast iron jobbies and quit using it after a few years – it’s just too small. It won’t hold enough wood chips to last long enough to do the job. It’s a large pain (in the hands, and arms) to add more chips if you put the box below the grill and you lose the smoke already produced when you open the grill to add chips. BTW the cast iron boxes do NOT last forever! The heat and moisture rusts them rapidly and they begin to flake away. Now I use a small stainless steel pan 8”x8”x1½” (probably a square cake pan – I got it at Target) with some holes (15 or 20) drilled in the bottom as the top and another as the bottom (no holes).This thing works well, is easy to clean, doesn’t rust, holds three times as many chips as the cast iron type and only cost about $8 total. There is an alternative to either of these methods. Make up some small (8”x8”) packets of chips sealed in heavy duty foil. Punch some holes (not too many) in the top and just throw one in under the grill during the preheating. If you run out of smoke throw another one in. No clean up!

sensible_man
When smoking food on the grill, the temperature should be low (225 degrees). Set your wood chip box on the grate and light that burner. leave the other burners off. The wood chips should be soaked in water for about an hour before using. You do not need smoke for the whole cooking process, just the first hour.

Louis
I assuming that the casta iron smokebox is an aftermarket "add-on" that didn't come with the grill. If I'm wrong on that, you can probably still try this. If it is meant for that grill, there's probably someplace to attach it -- but smoke is smoke. First of all, good that it's cast iron. An aluminum or other thing/flimsy metal could actually start to melt in a hot grill. I'd turn all three burners on, two on low the other on med-high. Soak your wood chips (you already knew that, I'm sure), drain off the excess water, place them in the smoke box and lay the box under the top-grill directly on the med-high burner or "coals" or heat diffuser (whichever way your grill is made). Place the food over the low-setting side so it will cool slower and be able to absorb smoke. Keep an eye on the wood chips and adjust the heat if they start to burn... a spray bottle of water is handy to keep around. Bon Appetit!