Write every chance you get. Write every day if you can. You should be thinking about writing or actually writing whenever you can.
Reading is so important. You should read every chance you get as well. Read good writing and read about writing. Balance fiction and non-fiction to help you improve your own writing.
Use your everyday observations to help you with coming up with surroundings, characters and situations for your stories. You can do this by keeping notebook with you, or if you’re not afraid of being stared at, use your phone and a recording app and pretend your on a call while you make note of what you see.
Check your writing for repetitive words and phrasing. If you find yourself using the same words repeatedly, refer to a thesaurus (Shift-F7 in Word) but don’t confound your reader with extravagant utterances and locutions that will flummox them. (Fancy, confusing words)
Know and use your grammar and punctuation rules. Your editor will thank you if your manuscript is as clean as possible when you send it off to him or her. In my first manuscript, I incorrectly placed all of the punctuation in my dialog outside of the quotation marks. That being said, when writing dialog, there are times to break with grammar rules in the voice of your characters. Just make sure it sounds natural and not convoluted.
Eliminate distractions like TV, social media, etc. If you need the Internet to do research, do it first or do it during a predetermined time. You want to avoid going down a rabbit hole during your research or hours of writing time can easily disappear.
Read works by highly successful authors in your genre. There are probably valid reasons why they are successful.
Read the classics. There is a reason that they are classics. I’ve found myself reading Doyle, Dickens and Hemingway this year and it has helped my writing immensely.
If you are new to writing, join a writers’ group so you can gain constructive feedback from the writing community and enjoy meeting other authors. I highly recommend Scribophile as it is a site that encourages you to give feedback in order to receive it.