where can I get a plan for a home made jig for installing knobs on my cabinets?

First of all, if you buy it, it's not home made. So you can make one, or you can buy one. To buy one, check the wookworking tools/equipment catalogs or websites, such as Woodworkerssupply.com, woodcraft.com, rockler.com, etc. To make one, cut a small square piece of 1/4" mdf or masonite. The square needs to be sized large enough to run about 1" over the proposed hole you will make in the cabinet door/drawer for the pull. Apply some 1/2" x 3/4" solid wood (pine, poplar, whatever) to 2 edges of this piece. Locate on the surface of the 1/4" stock where you want the hole for the pull and drill it in the jig (off the door). Place the jig on a corner of the door and voila, you have a hole locator for your cabinet pulls which will layout a hole on your doors in the same location for each one. To do a drawer front is similar, but you won't be able to work from a corner. You get the idea.
This site has tons of plans for jigs. http://www.woodworkersworkshop.com/resources/index.php?cat=102