Step 3: Doors!
Begin with some sort of hardwood -- oak, ash, maple, etc. Old flooring would be a good source, as would certain pallets. Alternately, poplar, frequently used in old door frames, is a good candidate. It is softer, but cuts and squares beautifully. Your source material should be somewhere around 3/4" thick.
Use the table saw to cut your stock into 1-1/4" strips. Set up a stop on the chop saw and cut the strips to length for the door frames. Every cabinet is different. You can size your doors various ways -- measuring the old doors, should you have them, is the easiest way. However, in my case, I was switching from sliding doors to swinging doors, which meant the old panels were irrelevant. So, I sized my frames 1/4" smaller than the opening all the way around, meaning the door frame was 1/2" shorter in each dimension that the opening.
Set up a dado blade on the table saw so that it comes halfway through the thickness of your stock. Make a simple cross cut jig with a miter gauge and two blocks of wood, clamping on a stop that prevents your notch from going too far. If you don't have a dado blade, you can make lap joints with a single blade a more passes; lots of passes with a chop saw with a depth gauge; using a handsaw and a chisel; or on a router table.
Once your joints are cut, lap the pieces with some wood glue, check square, and pop together with a brad nailer. Again, if you don't have a brad nailer, put them together with small screws, hand-tacked brads, or an electric stapler.
The stock I used already had finish on it, so I buzzed with 100 grit sandpaper so glue and new finish would stick to the surface. Also, sanding eased the edges, took burn marks form the saw off, and got everything nice and flush.
For the door panels, set a stop up on the chop saw so that every piece you cut is the exact same length. Feed shop scraps into it. I used pallet wood; wood from wine crates; crown and base molding; off-cuts from the table saw; hardwood flooring; and some poplar door frame scraps. This is a great way to use up old stock that's been laying around the shop. If you don't have access to a shop or deep scrap pile, poke around alleys, dumpsters, and construction sites for any kind of scrap wood. Since the strips are so small -- 17" long in this case -- it is easy to make use of throwaway pieces from dumpsters and elsewhere.
Take your strips and align with the corners of one end of one frame, lay down some glue, and tack into place. Do the same with the other end and work towards the middle. I did a mixture of random gaps; if you want no gaps, or consistent gaps, take the time to measure out how the pieces go together, dry-fitting and numbering them first.
Once together, put a coat of polyurethane over everything, back and front.