TL;DR - I found this book effective, you should read it instead of listening to me blather below:

https://www.amazon.com/Taking-Charge-Adult-Russell-Barkley-e...

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The biggest things, broadly, are:

Step 0: Get engaged medically - with diagnosis, collaboration, and medication. ADHD is not something you can manage on your own. It's literally "the disorder of not being able to fix the problems you can clearly see", so you need external feedback and assistance. Doctor(s), therapist, friends, family, co-workers, employer - gotta engage all of them.

Step 1: Figure out where the problems are. This means, before you make any changes, record when things go right / wrong and map out places that need change. (and I know, making ADHD people carefully analyze and wait it out is the most ridiculous thing ever but...)

Step 2: Change your environment and the way you live your life. Automate, externalize, or delegate all nonessential tasks and change your set/setting to accommodate you instead of struggling to behave. If it's not something you do without thought or effort, automate, externalize, or delegate it.

Step 3: Relax! You've made it, time to kick back and... Oh, no, sorry, continue to repeat steps 0-2 on a weekly/monthly/annual basis, largely for the rest of your life. It's a little bit like weight loss - getting there is only about 10% of the battle. Getting consistent about it is the other 90% and that's the part that clearly people with ADHD struggle with the most.

Anyway, it's really key to understand it as a process, a skill, and one that you can improve at over time. Don't let missing things or being unsuccessful get you down, just pick it back up and learn from where you made the problem. Don't blame yourself any more than you'd blame yourself for being colorblind or tall.

More about the changes in step 2:

a) I automate and delegate everything even more heavily than I did before. All bills go on autopayment, anything that can be set up to be routine is. Yard service, cleaning service, whatever it is, if it can take things off my plate, sold. Can't overstate how critical it is to not let your own execution problems trap you. Focus on strengths and accept your weaknesses.

b) Calendaring is absolutely essential. He recommends paper, I do it digitally, whatever works for you. I have things set to remind me, and I use it religiously. I have reminders for any recurring activity - daily, weekly, monthly, if it can't be automated, don't try to remember it, instead use reminders. It also must be with you 100% of the time. Literally chain it to your arm if you need to. Which sounds less ludicrous when you call it a 'smartwatch' but...

c) Externalize everything. Draw pictures, flowcharts, checklists, plans, or sketches; set timers, write notes, make boxes and organizers; put gold stars on a chart; whatever it takes, but you can't rely on your own brain to provide feedback. You need things to be concrete and outside of your own mind.

Most importantly, the things you need have to be at where you perform the task, not in the office. I have 3 whiteboards in my house and they are constantly full with relevant, localized info, plans, lists, schedules, etc. I use painter's tape and markers to label everything. Do not label the cats. They do not like it. I routinely go through ~3 rolls of tape a month.

d) You need your environment to fit you, not the other way around. If you can't focus at work, either make changes to where / how you work or find a new job. If your home/neighborhood/city is the wrong size, shape, or activity level, change it. It's that simple, and that serious. A significant undertaking and one that I'm definitely still working on.