I'll mostly just copy/paste since I posted this elsewhere a few days ago.
Short answer: https://rtalbert.org/gtd/ (I'm not that guy. That's just my favorite resource on the topic.)
Long answer: This is the organizational system I use for tasks at work and in daily life. Once I know what I need to get done and have it out of my head, it frees me up to (1) be deliberate about how I spend my time and (2) focus relatively distraction-free on whatever I'm doing at a given time, even if that's something simple like watching a movie without wondering whether there's something I forgot to do for work.
The author's goal really resonated with me: be okay not doing what you're not doing. It's not always about doing more. It's about deciding what you need to do, doing that, and then not having to stress all the time. The article is tailored to academics, which was where I worked at the time, but I still use it now that I've moved on, and I see no reason it is not generally applicable.
I know you asked for something simple, and this seems like a lot at first, but you don't do it all at once. Even the author of the article recommends that you start small. I spent years doing just the first few steps (mainly Collect, Process, and Simple Trusted System) without even attempting bigger-picture planning and review stuff, and it was still life-changing for me.
If any of this resonates with anyone reading, I'd recommend you give the first step a shot today. Keep it simple, start small, but actually start.
ADHD-specific info: I've used this for about 5 years, and for 4 of them I was undiagnosed, unmedicated, and mostly didn't know what ADHD was. Even then, it was incredibly helpful for me, but it definitely wasn't a panacea. Combined with meds, it's been really great for me. If anyone is curious about specifics of how I do things, I'm happy to elaborate as needed.
Bonus for ADHDers -- it's a set of articles you can use to put off whatever you're supposed to be doing! I'm quite sure that when I started, it was because I was trying to avoid something important like grading :)
Did a TM test for 5/3/1 for the first time last week. (I just bought Forever and figured out I should do these regularly along with some other mistakes I've been making.) Had to lower my Press by something like 15 pounds, which is a big decrease when you're already only around 100 lbs. Lowered squat a bit too.
This week, I'm really glad I did. It felt great to get a lot of nice, clean reps in. After eating a slice of humble pie last week, this felt like one of my better weeks in a while.