What advice did Mr. Henshaw give Leigh about becoming an author? Why would keeping a diary be one way of following this advice?

Mr. Henshaw advises Leigh that if he wants to become an author he needs to write. And although I can't figure out how to do it here, Mr. Henshaw underlines the word "write" twice.

Leigh decides a good way to get practice writing is to keep a journal. He has some trouble getting started, so Mr. Henshaw advises him to pretend he is writing a letter to someone. Leigh says, "I suppose I could pretend to write to Dad, but I used to write to him and he never answered." Leigh is very lonely since his parents divorce, even more so when his mother has to work longer hours at the catering company, so Leigh decides to pretend he is writing his diary to Mr. Henshaw.