I have a novel I have wanted to write ever since I was a child, but I know I do not have the skill to do so yet. What methods and sources do professional authors use to practice and hone their skills?

We sit our asses down and write. That is the only way you get practice.

At this stage I would suggest writing fan-fiction and publishing it on fan-fiction websites. You’ll be in a position where you don’t have to create an entire world, just characters and storylines. You can discover if you are better suited to writing short stories or longform. And you’ll get something very valuable: criticism. Some will be good, some will be bad, some will be batshit insane and not at all relevant, but you’ll learn to tell what is valid and what is not. And you will develop a nice thick skin. You’ll need that if you intend to actually publish something, and especially if you intend to professionally publish. Real world critics are not nice. Reviews on Amazon are not nice. Editors are not nice. You need to be able to take those punches, learn from them, or roll with them.