The old joke of "How to get to Carnegie Hall?" "Practice, practice, practice!" applies to most activities.
Do lots of writing and use grammar checkers, spell checkers, your significant other, whatever is available to you. Don't assume they will solve the problems but will assist you in recognizing them. Eventually, you begin to recognize problems on your own. Sometimes, because people are basically lazy, you will get that damn mis-spelling correct because you get tired of changing it -- yet again! Read as much well written material as you can. Try to find somebody who writes on a topic you like or care about (so you will be motivated to read) and try to pay attention to how they do it.
Hi, there, self,
There arre a lot of different things you may hear about poor grammar and usage. Most of them will reflect what the person commenting learned or has learned. My own personal take on this point is that poor grammar and bent usage will impede communication, but, not as much as lack of vocabulary and poor pronunciation. Of course, in this present context, pronunciation is rather irrelevant. So, continue to continue and damn the torpedoes.
We just had a orthographical reform in Germany which among some good reforms brought a lot of nonsense, in my view. So all spell checkers are more or less useless. All my orthographical abilities in all languages I know come from reading, reading, reading and sometimes from writiging and therefore looking up word in dictionaries (English has nice traps!).