This is most important in times of stress. Perhaps you just got misgendered, or had a difficult interaction with a family member who isn't supportive of your gender. Perhaps you're feeling especially fragile, or dysphoric.
Self-care might be the choice to spend alone time, which is sometimes hard to navigate or ask for if you are partnered. It might be as simple as taking a long bath, a nice nap, or taking the time to cook yourself a nice meal. It might be watching a movie, or playing a game, or getting a good workout. It might be taking a hike into nature, or a walk in a park. It could be calling a good friend, or having (virtual, these days) tea with someone who supports you.
The self-care choices you make are really personal to you - my self-care strategies probably won't work for you, and vice versa. Some of us know really well what our self-care strategies are. But if you don't, that's OK - you can figure it out.
Make a list of the things you love to do - the things that really nourish you. Then experiment. When you feel like you are emotionally fragile, go to that list, and choose what seems like a good thing for that moment. Then notice what happens. Does this thing help to bring you more back to yourself. Does it help you feel less fragile, or more emotionally stable?
And as you work with this, and grow your willingness to do self-care, you might find that you notice when you need it sooner than you did before, which can help those self-care strategies really work to bring you back into yourself.