When it comes to writing routines, the stock standard approach is write daily, make a routine of it, and stick at it almost every day.
I suggest the same approach, especially for beginners.
Begin by writing a daily diary. I religiously wrote a daily diary for eight years before I turned my full attention to story writing. It was just sit and write about my day, my thoughts and ideas, anything that came to mind. Over time I slowly started writing stories until I replaced diary writing with story writing.
In the diary I wrote about how other people may have seen the situation, or how I struggled to understand why people do what they do. I self reflected, pondered relationships and much more. There were no rules other than writing every day.
Most of it was crap and I cringe when I read it now, but that is not the point of writing a diary. The point was to strengthen the ability to take an idea within your head and get it onto paper. Eight years of doing that worked that process out for me. Now, although I do not consider myself a true expert, I know I am able to write ideas and stories onto paper (not literally these days though) better than most people.
So from my experience, I strongly suggest beginning your writing with a dairy or simply writing every single day. 100 word stories is one way to flex the writing muscle because even on a bad day, a 100 word story is possible.
BUT……….. while I am sure people will disagree here, now I have been writing for over thirty years, I am not sure a daily writing routine works for me.
Personally, I find I work better in bursts now. I have a base goal of writing about 10,000 words a month. That is one novel per year as a base goal, although I do not write novels because the energy required for a small potential result hurts my brain. Basically, I write to be read and my short works get read far more often than a novel will be.
I know the target, I understand the time it takes to get there, but rather than sit down each day and write 300 words a day (seriously 300 per day is not a lot) I will write a few times a week and write 1,000 to 2,000 words worth in longer sittings.
In short, I binge write. But when not writing I am thinking about writing and ideas and noting down story nuggets to work from when I do write.
It works for me.
What works for you may be different, but if you are beginning to write I strongly believe that daily writing is a must do thing. The brain is amazing, but writing is tough and requires your brain to work hard, so the more you do in the beginning, the better you will become.
When you have reached a certain level of expertise, you can begin working a routine that fits you on a personal level. Just realize that the learning process takes years, not months, of daily writing. If you have not written daily for at least five years, you are still a learner.
Writing is tough, it takes time, work hard in the beginning and reap the rewards of being a good writer later on. Routines will absolutely help you master the art, especially in the beginning years. When you have a strong grasp on writing, well, by then you will have figured out the routines that work best for you. Just remember, in order to be a writer you must write at some point.