Before the age of institutionalised music schools, who taught pianists how to play?
Other musicians. For most of the famous pianists, their fathers were their first teachers.
" In the fourth year of his age his father, for a game as it were, began to teach him a few minuets and pieces at the clavier."
Beethoven's first music teacher was his father. [...] Some time after 1779, Beethoven began his studies with his most important teacher in Bonn, Christian Gottlob Neefe, who was appointed the Court's Organist in that year.
At age six, Franz began listening attentively to his father's piano playing and showed an interest in both sacred and Romani music. Adam began teaching him the piano at age seven, and Franz began composing in an elementary manner when he was eight.
Johann Jakob gave his son his first musical training. He studied piano from the age of seven with Otto Friedrich Willibald Cossel.
And, of course, Rachmaninoff himself.
When he was four, his mother gave him casual piano lessons, but it was his paternal grandfather, Arkady Alexandrovich, who brought Anna Ornatskaya, a teacher from Saint Petersburg, to teach Sergei in 1882.
I'm sure you can learn this by yourself. But for the piano as for many other skills, learning from somebody who already knows how to do it can be immensely helpful at the very least.
You don't have to re-invent the wheel every time you want to learn a new skill.