Spelling and grammar are to communication what paint and trim are to architecture. Communication is the greater goal.
There are probably as many definitions for good speech and writing as there are people discussing the topic.
I suggest people who wish to speak with confidence and authority consider joining a local Toastmaster's group. There you will find others desiring speaking skills in all levels of attainment. They offer booklets of hint and tricks, what to avoid. They will teach you devices and gimmicks and punish you with the infamous "ah!" counter. They tend to meet t lunch or after work. Check your local listings or Google "Toastmaster" for you hometown.
If you wish a more formal setting for learning speech and speaking, consult you local community college. There are may options including rhetoric and debate.
Writing is communication just like speech. You use words in a row selected and composed much like a composer writes music. I suggest and desiring to be better writers take a couple of night school courses. If the course material is any good it will discuss, compare, and contrast narrative, technical, illustrative, etc in general and also explore rhetoric, research materials (and how to find them and employ them.) This is not to be confused with composition and how to write a simple declarative sentence. Writing conveys whole ideas, concepts, and arguments in complete works whereas composition is used to manufacture the sentences, to organize them into paragraphs: the essential parts of a written work. More or less. You might say writing is strategy and composition is tactics.
You can't learn either over night. If you have a desire to learn writing you must spend the time to learn the skills and a few community college courses are the best way to do it. Sometime early in the course you will be asked to write a little of yourself. This is the time to tell your instructor your reasons for taking a writing or composition course and ask to be to forgiven your problems with grammar and spelling which you are working on in parallel along with taking the course. Your journey might take a couple of years. Time well spent for a younger man wishing to further himself.
You will find that an ability to speak and write will promote better understanding of yourself and others, advance you in your trade or profession, render you more resistant to bullshit and propaganda etc. Basic skills in speech writing are as valuable to a citizen as welding is to a metalworker.
Let's add one more skill to the mix: sketching and drawing. in 1978 I took a couple of night courses in basic sketching and drawing. An ability of expressing intelligibly on paper the concepts in your mind advances not only your own understanding but the understanding of your co-workers and colleagues.
Good spelling and grammar are certainly important. And ability to communicate in speech and on paper is as oxygen to communication.