Honestly, "just doing" is exactly how I would recommend you start developing a schedule.

I know that doesn't really answer your question, but truth be told, good schedules that you're able to consistently act on usually require a long period of adjustment while you figure out what works for you at this point in your life, and as your life changes, you'll have to adjust your schedules accordingly. It's not the case that one first develops a schedules and then follows through on adhering to it. It's often more effective to just start doing something, anything, and fitting it into your life as it currently is. It sounds like you've already done that, and by "just doing" you've already done most of the work.

Consistency will ebb and flow, there's nothing you can do about that. Starting is often the hardest part. If you're able to work some amount of progress into an arbitrary time period, then you're already doing the thing. Don't worry if it's not every day - maybe you can only practice 3-5 days of the week. That's totally fine! Just start setting easy goals for yourself, and if you find yourself consistently achieving them, then you can keep adjusting your goals until you find the sweet spot where you're making enough progress to feel good about, but not burning yourself out.

Hopefully that's somewhat helpful. Keeping a schedule is something I personally struggle with too so I feel ya. Try not to beat yourself up if you're falling short of your goals - our hobbies are supposed to enrich our lives, not define them. Good luck on your journey and happy drawing!