Well there's various ways to do it but I'll describe how the big box we used works.
There's some flue pipe buried underneath it that goes off to the side, the ground slopes down so it eventually come out of the ground. At the opening of the flue pipe we build a little fire, don't need much. All the smoke goes up the flue into the box. Like I said a little fire, you want to keep this fire just smoldering. We usually use a combination of wood that has been soaking in water and some dry stuff. You'll want apple, cherry, maple or similar wood.
The key is the temperature inside of box and length of time but that's ancient Chinese secret and I would have to kill you if I told you that.
It's going to vary according to the meat and truthfully it's been a while so I forget but if I remember correctly for the kielbasi we smoked them for about 5 to 6 hours at 120 then bring the temp up to 160 right at the end for about 15 to 30 minutes.