Most of this advice is good. It can be boiled down to, if you feel awful, try changing one simple thing.
I will add, don't pressure yourself too much about anything. Also check to see if these things make you feel better or accomplish a practical purpose. If so, keep doing them. If not, quit and try something else. Don't try to force yourself to look or act "normal" if it doesn't help or actually makes you feel worse. You've got better things to do with that spoon.
Beyond that ...
Are you hydrated? If not, have a glass of water.
Have you eaten in the past three hours? If not, get some food — something with protein, not just simple carbs. Perhaps some nuts or hummus?
Yes, bodies need regular fuel to keep going. These are not optional as long as you're alive and wish to stay that way. Falling short on these turns into an emergency pretty fast.
If you are okay and someone else is not, the shortlist is all the physical stuff: make sure they are warm, dry, safe, not bleeding, watered, and fed. That can't fix everything, but it definitely keeps a lot of problems from getting worse and it makes everything else easier to address. Also address the emotional issues: provide reassurance, contact comfort if desired, support for decisions or practicalities they're not up to handling at the moment.
Have you showered in the past day? If not, take a shower right now.
If daytime: are you dressed? If not, put on clean clothes that aren’t pajamas. Give yourself permission to wear something special, whether it’s a funny t-shirt or a pretty dress.
These are examples of pretending to be normal in ways that may not help you when you are wrecked. By all means try them. But if they don't work for you, don't beat yourself up. Frex, oily hair/skin needs to be washed frequently but that can be bad for dry hair/skin. If you get sick or your hair becomes unmanageable when you don't wash every day, then hygiene is really important for you. But if your skin and hair are better without daily scrubbing and you're not getting sick, figure out some other schedule. If you're not going out, there's no reason you can't wear jammies all day if it makes you feel better. Are you unhappy in regular clothes? Try putting on pajamas and taking a nap or watching cartoons.
The reason people quit doing this stuff when grieving, for instance, is the same as your hands getting cold when you're hypothermic: it's an emergency reflex routing scarce energy to critical core circuits. Losing a hand won't kill you; losing your heart will. Going without a shower won't kill you; not drinking water will. As long as you're prioritizing in that order, something is actually still working exactly as it should. Go you! When you feel better, you can do more stuff again.
If you are okay and someone else is not, then talk about whether they want to be prompted because they forget to do this stuff, or whether they just don't have the energy and need to not be nagged about it. Sometimes just accepting that they are not okay right now is more helpful than pestering them to pretend they are okay.
Do you feel unattractive?
Here you need to be careful. If you have supportive friends, great, use them. But lots of people don't. This is especially true if you have traits that people like to pick on. You may find a support group online or in person. Also positive affirmations work without needing anyone else; the trick is to find ones you can believe right now. Bad tape is super tedious to fix but it's like debugging code line by line: every time you contradict it, you get one step closer to erasing it.
If you are okay and someone else is not, listen for the bad tape and help them overwrite it with something nicer and saner. Be very careful because compliments are stressful for some people who have bad tape, anxiety, etc. Say one nice, true thing about them and move along. ("I appreciate that you're always on time. Today's first task is X, so let's start...") You can also disagree with ("I'm so stupid!" "I think you're pretty smart, because X.") or correct ("All my projects fail!" "Last week your X went well, so sometimes you succeed.") distorted thought patterns.
Have you seen a therapist in the past few days? If not, hang on until your next therapy visit and talk through things then.
This is another risky one. First, therapy doesn't work for everyone. Not all styles work for all people or problems too. So don't assume it will help you. Research to pick a style that's supposed to work for your issues and personality. Give it a try and find out.
If you are in therapy and it is working, yay! You don't have to wait for this kind of help, although if you're not an emergency case, you may have to wait for YOUR therapist. Meanwhile, don't try to flounder through alone if you feel like you can't. People get hurt that way. Call a friend or relative. Call a helpline; there are general ones and thematic ones for specific stuff like suicide or self-harm. Use an online forum, general or thematic, to connect with other people going through sticky shit. Try to find safe, effective ways of reaching out when you need help.
Are you okay now and want to help people who aren't? You can be a big hero by putting yourself on somebody's call list for when they need support. Or you could volunteer at a helpline -- most of them offer training too. Often what people need most is not professional help or someone to fix their problems. It's a hug, a kind word, a sympathetic ear, validation, a shoulder to cry on. Most folks can do at least one of those things.
Have you waited a week? Sometimes our perception of life is skewed, and we can’t even tell that we’re not thinking clearly, and there’s no obvious external cause. It happens. Keep yourself going for a full week, whatever it takes, and see if you still feel the same way then.
You’ve made it this far, and you will make it through. You are stronger than you think.
Risky again. Waiting is great, if you can. Lots of people can't, especially without support. And not everyone DOES make it through. Physical and mental illnesses kill people all the time. Do the best you can. Try to find and use a support network, because that improves your chances of overcoming difficulties. Try not to blame yourself if things still suck anyway. Everyone has things they can cope with and things they can't. Getting run over by life doesn't make you a bad person.
Finally, for any kind of problem-solving, it helps to measure the results. What is bugging you? How will you try to fix it? What would success look like? Try your solution and note what happens. Do you feel better? If so, yay! If not, try again; after several repetitions of failure, you should probably try a different solution.
When you feel okay, you can probably do this in your head. When you feel like crap, it can be super hard to distinguish any kind of improvement, let alone remember what helped the last time. Try using a mood tracker to record what you are doing and whether it helps. Keep a list of coping skills that help when you feel bad. Then you don't have to think them up while you are stressed, you can just look at your list.