A good starting point would be to buy a bunch of pipe cleaners, bristle and regular. Get some spirits: Everclear or Golden Grain alcohol from your local liquor store. But if you already have some vodka, rum, or whiskey, you can use that too. Dip the very end of the bristle cleaner in the alcohol and run it through (back and forth to scrub) the stem and shank, being careful to not let the alcohol get on the finish of the pipe. This will loosen up any tars that may be in there. Follow up with a plain pipe cleaner to soak up the tar/alcohol gunk. Repeat until the regular pipe cleaner comes out clean or close to it.

You may want to ream the bowls if they have built up carbon much thicker than a dime. Don't use a pocket knife or you may damage the inside of the bowl. Some people can do it, but it's much safer to use a pipe reamer. I like these reamers:
https://www.pipesandcigars.com/p/dec...5127/#p-203521