A lot depends on what tools you have and what sort of doors you need (or want). I redid all our interior doors in a former home using construction lumber and shelf boards. They were pretty simple four panel raised panel doors. The early ones had a 1/4 groove all around the inside edges of the openings. The panels were raised on both sides and went in them as did splines at the joints. I added dowels at the joints and the doors worked. Later I went to mortise and tenons and those doors were a bit more solid. I raised the panels on my table saw but it can be done with router bits. You could make simple doors like this with a planner, track saw, router, and drill.
I've made mortise and tenon joints by drilling and chiseling (least favorite), plunge routering, and with a hollow chisel mortiser (currently most favorite, hoping for a domino some day). I've used slip tenons and made traditional tenons. For big pieces I like to put a dado blade on my radial arm saw and then trim with a shoulder plane. I think the latter is the most important tool if you want to make normal tenons. Too hard to cut to precise size, much easier to make a little oversize and trim to fit.