I feel like a lot of the classic tools for 'becoming a writer' may not apply to those that just want to sell books. Especially in today's attention deficit market. If you're writing 'for yourself', then it definitely behooves one to look at more tried and true methods for honing your craft.
I'll always recommend Stephen King's On Writing to anyone looking to get started. I almost consider the first half of the book more useful than the second half, since part one deals with his life and the challenges of trying to become a writer in an impoverished home. It can be taken as a metaphor for persevering and being pragmatic regardless of what your chosen creative endeavor may be.
I think mental fortitude in any creative effort is the most important element for anyone looking to do this sort of thing. If you're bad, you CAN, hypothetically, become good with time and effort. If you don't labor to become good, you'll always be bad.
In one of my stories, I named the protagonist's horse 'Resolve'. That was personal and it's a constant reminder to me that I could always write more and keep at it.