Here is a pic of my UDS
Get a clean drum, with no liner, or you will need to burn out and grind out the liner and that's just too much work for a guy like me!
once you have a clean drum you will need 1-2 22.5 weber kettle grates. if you have an old Weber kettle grill you can use the parts. I made two grates one at the top of the barrel and one about level with the temp guage.
in the bottom drill 3 1" holes. on 1 hole you can thread some standard black plumbing pipe nipple into that and attach a ball valve to control air flow. on mine i added a elbow and 24" extension, so, my fat *** doesn't need to bend down. if you position it correctly you can be within arms length to adjust the air flow. the other two you can either put in nipples with screw on caps or just get some fridge magnets and cover them up.
create a charcoal basket using some expanded metal and the bottom charcoal grate of a Weber. on mine i cut out the top and bent the lip of the cut out in and created a ash pan. then i put a few bricks in and set my basket on the bricks. just need a few inches to allow air flow under the basket and room for the ash to drop.
for the top a Weber Kettle fits perfect. you can get a temp guage and place it a few inches below the bottom cooking grate.
when you start the fire fill your charcoal basket with real lump charcoal. get about 10 regular charcoal briquettes lit in a charcoal chimney until they are white and glowing. once you have that you put your full basket into the drum and position it properly. make sure all three 1" air holes are open.
once the basket is set you better have all your food ready. i suggest using a digital probe to keep track of the temps. that way you don't need to open it up until your food is done. put the 10 lit pieces of charcoal in the middle of the basket on top. put the food in and set the probes and put the cover on. once the temp gets to about 180 cover 1 air hole at 200 cover the other air hole. use the ball valve to adjust down to the desired temp.
i find mine goes easily all night at about 240-250 depending on the weather. I've done 14hr brisket smokes on 1 basket and had about 35-40% of the basket left over!!! once your smoke is done you can shut down the air and the fire will die and the charcoal can be use the next smoke!
you can use a water pan if you want, but, i didn't find it necessary. on mine pork shoulder takes about 9-10hrs to get to 205 degrees and brisket is about 14hrs depending on the time i start and the weather.
is the ULTIMATE cheap and consistent smoker. now when i need to cook a bunch of meat i put it on at night and go to bed. the temp stays steady and the digital temp remote will warn you once it's done. some guys put an additional prob in thru a potato to track the temps in the smoker, but, i don't think thats needed. limited consistent air flow will control the temps.