Resist the urge to over-complicate things. With ADHD, it's really easy to hyperfocus and end up building a "beautiful" system that doesn't work at all for you. Then you give up and start all over. So instead, pick small things that you can incorporate into routines, which are a saving grace especially with ADHD - just include enough space for a bit of flexibility so it doesn't get stale/boring.
For instance, I have a morning routine which ensures I'm "presentable"/etc. When I start work I immediately create the day's note, go to the previous day and review, copy over any ongoing tasks, etc. My day note is the same thing every day: Things I did, Things I need to do, Meeting notes (important meeting notes get extracted to their own file), Random notes. Then setting in to work. Evenings are bit more flexible and the weekends tend to be the wild west, bit of a reset so I don't feel "trapped" in a cycle, etc.
I do struggle with weekly/monthly or longer intermittent routines. Even stuff like doing bills (automated as much as possible), re-ordering prescriptions, etc. So it's always a process.
Last thing so as not to go too long - not everyone runs into this, but in case you've gotten down on yourself at times and now realize it might be ADHD, give your self a break / forgive yourself. Same thing going forward. Not an excuse, not continuing to seek improvement, but realizing that when you stumble, there is a reason and it may not be something you can actually control. Reflect on what you could do to prevent it in the future, but do it without self-blame or criticism. Be kind to yourself, in other words.