A BWB (Boiling Water Bath) canner is really just a big stew-pot with a rack to hold and lift the jars with.
Here's my 21 quart "big blue monster". (ignore all the junk on the table in the background ha ha)
It's actually brand new. I wore my old one out and bought this one to replace it but haven't used it yet. It's a "Back to Basic's" brand.
To process in a BWB canner (as opposed to a pressure canner) you just fill your canner with water until the water comes to at least 1 inch over the top of the jars (you must keep it at this depth so keep a kettle boiling to compensate for evaporation) and bring to a boil.
I load my jars before I fill the canner, then I know I have the correct depth, then leave the jars in while it's coming to a boil. Once boiling, I boil my clean jars for 15 minutes to sterilize them while I'm preparing my food for canning, and then only remove the jars from the canner when I'm ready to fill them. (Remove with a jar lifter, then pour the water out. Don't pour it back in the canner or it'll over-flow and put your fire out when you put the filled jars in to process).
Then boil your packed jars according to the reccomended time on the recipe and adjust for elevation (chart and elevation data for your location available from your local extension office).
Once the processing time has been reached, take them out and set them on tea-towels on the counter, NOT TOUCHING AND OUT OF A DRAFT (Uneven cooling can cause the jars to break and make a bloody mess of your kitchen) and let them cool. Listen for the tell-tale "Plink" of the lids as they seal.
One of my fondest memories of my childhood is when Mom would spend all day canning and then to lay there at night listening to the 'Plink' 'Plink' of the jars sealing.
~ Raven ~