Newton taught himself Descartes's Geometry, on his own. It was not without any blockers. There is evidence that he got stuck at certain pages, and he would trace his steps back, start again, and go farther on his next attempt. This way, he mastered Descartes' mathematical works through one's own efforts, without a teacher.
He bought Descartes's Geometry & read it by himself Conduitt continued in language very similar to that in the DeMoivre account, when he was got over 2 or 3 pages he could understand no farther
than he began again & got 3 or 4 pages farther till he came to another difficult place, than he began again & advanced farther & continued so doing till he made himself Master of the whole without having the least light or instruction from anybody.
-- Never at Rest: A Biography of Isaac Newton
Also, Newton would plunge directly in and master many other mathematical topics completely on his own:
Newton's own notes agree with the accounts of Conduitt and DeMoivre that he plunged straight into modern analysis with no appreciable background in classical geometry . They agree as well with the centrality given Descartes. Schooten's second Latin edition of the Geometry, with its wealth of additional commentaries, was his basic text, supplemented by Schooten's Miscellanies, Viete's works, Oughtred's algebra (the Clavis Newton mentioned), and Wallis's Arithmetica infinitorum. In roughly a year, without benefit of instruction, he mastered the entire achievement of seventeenth century analysis and began to break new ground.
Newton held the capacity to teach oneself mathematics through one's own inclination and industry at high regard:
Both accounts agree in making Newton an autodidact in mathematics, as he was in natural philosophy. Nearly twenty years later, when he was recommending Edward Paget for the position of mathematical master at Christ's Hospital, Newton probably had his own experience in mind as he specified Paget's qualifications. Paget understood the several branches of mathematics, he said, "which is the surest character of a true Mathematicall Genius, learned these of his owne inclination, & by his owne industry without a Teacher." There had been even less mathematics in the university than natural philosophy; not surprisingly , no stories survive of undergraduates being stirred by Descartes's Geometry.
Therefore, Newton's chief source of learning was acquiring skills by working through mathematical texts entirely on one's own; and he respected people who took a similar approach.