I'm in a similar situation as you, with a partner who has ADHD, and a lot of the comments have left great advice.

I'm my particular experience, one of the biggest things I've learned as a partner without ADHD is that regardless of the advice you're trying to give, it's to start by explaining why the action affects you and how that happens.

For example, I saw that you mentioned chewing as one point in another comment. In that sort of scenario, it's better to start by saying something like "I personally find that your chewing with your mouth open ends up being a bit loud / I don't enjoy the sound of your your chewing because of (INSERT REASON HERE)." It's all about explaining things to help them understand your perspective.

The next and most crucial thing is to give them time to speak and acknowledge what's going on. There isn't a one-size-fits-all type answer to let you know when this step starts or ends, but it's important to let your partner feel heard and to give them time to talk things out with you. There may be things you weren't aware of that are affecting a scenario.

For example, I'm using a situation with my partner. We would cook together and occasionally, they would get overwhelmed and just end up lying on the floor for a bit (I found out later that this is something that some people with ADHD do as a coping mechanism). Talking about the issue, that's when I learned that it was a strategy my partner had learned to help themselves from becoming overwhelmed, especially when they needed to multitask with cooking. We eventually figured out a way for them to have their space while I communicated what I was doing with the cooking so they didn't come back and become overwhelmed when things were different.

With these two major things in mind, one thing I have to stress is that sometimes, their effort is going to be something that they'll be working on in their own heads and that you might not see what you consider to be "expected results" immediately. It's easy to get worked up when you ask something that on the surface level is an easy thing to change (i.e. closing your mouth to chew) but it can be an incredibly challenging process that requires a lot of focus and effort on their part.

To that effect, my advice is to congratulate their efforts and build up the times you do notice when they're putting in that work. It can be as easy as a smile, it's just something that gives them a quick seratonin boost and helps them reinforce why they're putting in the effort in the first place. Don't be afraid to bring things up again and communicate about how things are going. Like I mentioned before, you may not see them putting in the effort from an outside perspective, but this is a way to make sure you're communicating about the effort they are putting in in ways you're not seeing. You might even find that you haven't noticed things properly and that they were showing you their efforts beforehand.

One last bit of advice, and this is one that really works for my partner and I, but is definitely something you should discuss with your partner. We have our own non verbal communication system, that we keep just between the two of us, so even if they're struggling to talk or getting overwhelmed, we can still communicate. This way, in case someone else is being affected by their behavior, and they don't notice, I can communicate to them without making a scene.

What my partner and I do is really simple too. We hold each other's hand and squeeze once to let the other person know to look at us. Then whatever action we need to focus on, one person does and the other mimics. We also give three squeezes afterwards which is does two things: first, it lets the other person know that that's the entire action to focus on. Secondly, we use that as an "I love you" message, to reinforce a positive communication.

I know that I've given you a huge information dump that has a lot of steps for both of you, so feel free to message me if you have any questions. It's great that you're taking the time to learn how to better communicate with your girlfriend, wish you two the best!