Velazquez’s teacher wrote a book about him. Or rather, he wrote a book about painters and included in it his best student (and son-in-law).
As soon as you hear of this book you run to get a copy. Imagine: here an old master painter, who taught one of the world’s greatest painters from the time he was a boy (Velazquez lived with him), tells how it was. What was his secret? What did he tell the boy? How did Velazquez learn so well?

Alas!—he gives none of the answers to those questions. His book makes disappointing reading. The most he says is that the young Velazquez drew a lot and made many studies from life. That is no revelation.

Pacheco, the old teacher, was himself a mediocre painter. And though he saw how good his son-in-law was (who couldn’t?) and was proud of him the rest of his days, Pacheco wasn’t really very perceptive. He was full of conventional thought and practice. His most important contribution to Velazquez’s development seems to have been his allowing him to go his own way (which is not a little one).
That and the tertulias or regular get-togethers of local artists and philosophers Pacheco hosted at home. Like Michelangelo at the table of Lorenzo the Magnificent, the young Velazquez sat there and heard the talk of clever and well-educated men. Those tertulias were his college. They opened his mind.

Later Pacheco pushed Velazquez to try for the position of court painter in Madrid and used his influence to help him along.