According to the smoker size, you can decide the size of the tube. The normal length is 6" and 12" and the traditional shape is circular, square and triangle.
Its dead simple to use, add pellets(or maybe chips) to the tube, sit it up vertically, light and let it start burning well.
Once it is burning well, set it on its side, add your food, and close the grill. 

The Details of stainless steel Smoker tube:
Material: 304 Stainless Steel, can be customized.
Color: Silver
Length: 6'', 12'', 18'' can be customized.
6" Tube Smoker produces smoke for up to 2 hours during Cold Smoking and while supplementing smoke
12" Tube Smoker produces smoke for up to 4 hours during Cold Smoking and while supplementing smoke.

Features:
1. Great for both hot and cold smoking.
2. Operate easily and stop your nightmare of cleaning.
3. Just insert your favorite pellets, light the pellets, then place on the grill grates.
4. Designed with maximum air penetration to ensure a constant burn.
5. Works with electric, gas, charcoal or pellet grill.
6. Compatible with most wood pellets.
7. Unlock tantalizing and unforgettable smoked flavors in your meats, beef, cheese, fish, nuts, vegetables and more.
8. Plastic film package, white box individually packaged

How it works:

Step 1: Fill up the Smoking Pellet Smoker Tube with your favorite brand pellets.
Step 2: Light the filled pellets with a propane torch or butane lighter.
Step 3: Allow to stand and burn for roughly 10 minutes before extinguishing the flame.
Step 4: Begin smoking your meat, cheese, or vegetables for up to 5 hours and enjoy!