My girlfriend and I both have ADHD, but meditation has always been easy for me, especially at night.
You don't have to do anything special, it can be while your in bed before sleep, for some people it's just helpful to create a ritual or activity around it to occupy the part of your brain that's liable to fixate on stress.
I've tried sitting with her for a guided meditation app she likes, but it makes it harder for me. Sometimes I feel like Ron Swanson talking outside the meditation class. "I have no idea what these guys are doing, my mind was completely blank"
Absolutely. The most useful "habbit" I have for managing my ADHD is being brutally honest with myself at all times.
If you learn to reflect on your patterns and feelings, you start getting a good idea early on when you're gonna be in one of those dysfunction days.
Best thing I've found to do about it is "take the day off". If I know everything is gonna be uphill in a unique way, I take it easy. If I'm at work, I try to focus on the most accessible micro-tasks, or "tedious" things that take zero cognitive work.
For me those days are a sign of burnout and I know little will be accomplished if I force myself to overwork that day anyway. So I prioritize resting my brain. Sometimes it means doing nothing at all, sometimes it means video games or folling around with in GarageBand with a keyboard and bass.
Letting your brain do whatever random bullshit it's craving can be just as restful as doing nothing. Sometimes these days can actually be really productive for my hobbies, or housework, or spouse time, just depending on what my brain wants.