If you want to build a trotline you will need to go wherever you purchase your fishing supplies and get the following:
 
Approximately Fifty yards of a heavy cording.
About twenty-five swivels
Fifty clamps
Approximately fifty feet of a lighter weight cording
Twenty-five hooks

Start your trotline by allowing about ten to fifteen feet at each end for tying your line off.  Then continue by putting in a clamp, swivel, then another clamp, swivel, each spaced a few feet apart, so that your lines will not all end up in a tangle.  Make drop lines about two feet long (you can make them longer if you need to--for instance, if you are fishing for catfish in deeper water and want to fish the bottom) from you light weight line for each hook you want to have on your trotline.  Make a loop in the end of each one, and put a hook onto each one.  

Next, attach your drop lines, one each to a swivel until all are attached.  To set your trotline, you will need to bait your hooks, and find yourself a good spot where you will not be in the way of traffic.  Tie your line to a tree off to the side.  Run the line out, and weigh it down with a weight if you need to.  Be sure to come back to check your line just after dusk or early in the mornings for best results.

To get the full "Trotline Fishing For Catfish" article you'll need to download it here.