Self care is a wanky phrase but it just means doing things for yourself. That includes:
Washing regularly (use soap or shampoo you like the smell of) and cutting your nails regularly.
Sleeping enough and changing your sheets regularly.
Eating a diet that balances the nutrients you need to live vs the effort you are willing to put in (some people enjoy cooking or baking, some like takeaways, most of us like a bit of both at times but are also happy to have a tin of soup and enjoy the minimal tidy up).
Exercise. Necessary for physical and mental health. Can be as simple as walking the kids to school.
Spending fun time with your immediate family, e.g. watching Hey Duggee with your children.
Going outside. Walking to the shops, even hanging the washing out is something that I get a bit of pleasure from.
Spending time with the people you care about.
Doing a job you love and/or you feel has value. We all have to spend a lot of time working and you need to be getting something out of it, whether that is your friendships with colleagues, satisfaction that you are doing your job to the best of your abilities, or knowledge that your job is 'important' (I don't mean in a big boss way, I mean in a 'what I do has a positive impact on other people' way so jobs as diverse as being a carer or an engineer or working in the costume department in a theatre all impŕove the lives of other people).
Doing leisure activities that you enjoy. DH loves taking DD to her football games because he has a gossip with the other parents, they are out in the fresh air, and he's doing his thing with DD.
You can wank up 'self care' into a way to spend excessive amounts of time on expensive activities (e.g. MAMILs) but at its core it can be pretty simple things and if you can't see how to do it that in itself shows you are close to breaking point.