How do you teach yourself yourself? Zen is not psychotherapy, there's some confusion about that at times, but it has similarities and psychotherapy has origins with Zen to an extent (S.Freud and C. Jung were aware of Zen, and Jung in particular read heavily on it), but lets use therapy as an example. Can you do self therapy? That would involve self analyzing every thought you have - super honestly. People, yourself included lie to themselves, and omit important thoughts and feelings from their conscience all the time. So self therapy would never be as good, because someone else can catch you out as it were about things you try to hide from yourself.
In Zen training the same applies, you will not notice every thought, you shouldn't try, and you can't, you can't analyze the source of yourself honestly by yourself. So in Zen that's the role of a teacher, in multiple ways a teacher is someone to bounce off who will not let you lie. Imagine playing soccer without referees, you can have a game of sorts, but a referee is impartial to who is winning or losing (please don't respond with critiques on that game from last year's cup), a referee keeps the game flowing until it's outcome. That's why someone should seek out a teacher in Zen.
Going back to noticing every thought, someone might say but I can. Actually no. In Zen for many centuries we have the theory of San-Nen which Yung adopted into Psychotherapy, San-nen is how our mind works in concert with our body and surroundings building our reality - or illusions. Right now you have three types of awareness, one is reflex you don't notice your eyes crossing this screen as you read, another reads, and another makes sense of the words and thinks about what you're reading here. This has been proven clinically, and it's shown that for example in a simple yes\no question you decide yes or no moments before you become aware of the answer you will give. this is going on constantly, you are hearing sounds you heard milliseconds ago after your brain has picked them up and anylized if they are important enough or not.
So an important part of our conscienceness is each other, and the more informed we are by others therefore who have already walked the road we walk before us, the better we walk that road by ourselves.
That's what we've been doing for thousands of years in what we call Zen. And it works. And the Zen masters of old are very explicit, they trained to high degrees, some possibly becoming superhuman as a result, self teaching is not possible and in fact can lead down dangerous paths, such as self delusion, self harm and harming others. The mind is not a toy. A deluded person thinks they can control everything, and isn't that why you are seeking the Zen way? To overcome delusion and to responsibly bring out the best of yourself.