If you cut two pieces, try to fit them together, and they are gapped, or if you have to force them into alignment, or if the pieces will not sit flat together, don’t despair. In my experience, a failed joint is never the result of a single error, but rather a series of small errors that added up to produce the final result. To eliminate the error, it helps to identify where the error occurred, and this is best accomplished by going slow and observing what you are doing.
Common error is cutting the wood right on the line rather than to the “fat” side of the line. It is easy to remove excess wood to make a part fit, but it is impossible to put wood back on to make a part fit. If you cut right on the line, and you make a mistake, the part will be too small and you will have no choice but to start over. But if you cut to the fat side of the line, and you make a mistake, you can shave a bit of wood off of the part to make it fit. As you cut to the fat side of the line, I have found that it is helpful to cut right next to the line rather than exactly on it, then shave into the line. As you shave into the line, you should test the fit of your parts frequently rather than waiting until you have cut them both all the way to the line. It takes a few practice strokes to train your eye to be able to discern the difference between “almost there” and “there.” And I have found that if I test the fit of my parts frequently, I develop my skills more quickly than if I test the fit of my parts infrequently.
In fact, a good exercise to practice is to take a couple of pieces of scrap, plane or sand them flat, then attempt to join them edge to edge. As you push the pieces together, examine the contact points between the pieces. Are there places where the pieces don’t touch? If so, remove a bit of wood from the high spot and try again. I find that practicing this for 15 or 20 minutes helps me develop my sense of what is flat and what is not.
With practice, you will come to find that a good joint is not a mystery, and in fact, you will find that with practice, making a good joint is almost anticlimactic. Instead of relying on force to make parts mate, you are relying on good surface preparation. And if your surfaces are well prepared, your parts will mate easily and stay mated, even without fastening. This, more than anything else, makes the process of assembly relaxing rather than stressful. Instead of being a test of your skills, the act of assembly becomes a confirmation of your good work.