I'm a cis man. I've never properly learned to groom myself or take care of my things from simple instructions. Here's a bunch of lists of things I've learned over time so that you can avoid mistakes and painful trial-and-error.

Disclaimer: Use the information on this site at your own risk. None of the authors, contributors, administrators, vandals, or anyone else connected with this WikiBlog, in any way whatsoever, can be responsible for your use of the information contained in or linked from these web pages.

General

Read the damn instructions of things you're using. They're more complex/detailed than you think. You know nothing.
Always leave spare time to automate and learn new things. Never become so busy that you cannot do this. Then you are just a machine. Make machines do your bidding.
Make sure to always be open to being wrong and thereby able to improve your life.
Get a skill. One that can help others, where you can earn money. Get good at it.
Get rid of rarely used things. Just pay someone to rent said thing. It saves you time, space, and maintenance costs. Unless of course you really get joy out of ownership.
Have some basic tools: a hammer, a screwdriver with various types and sizes of bits, a wrench.

Personal hygiene

Use conditioner/balsam after shampooing your hair. This prevents fraying.
Use skin moisturizer if you have dry skin.
Use an electric razor (OneBlade) if manual razors irritate the skin. They're not as close to the skin as razors but it's good enough.
Wrap the blade(s) of a normal razor in a towel after use, it will dry faster and rust less, and lose its edge more slowly.
Shower once per day. In the mornings ideally. You're more smelly than you think.
Clean your teeth at least once a day. In this order: Brush, floss, rinse.
Wait a little before brushing after having eaten something acidic.
Floss by pushing the string against one tooth, then pull out of the slit. This essentially scrapes the side of the tooth. Sliding the wire through the gap is ineffective.
Do NOT use alcohol-based mouth rinse. Alcohol is hygroscopic and dries out your mouth. It is a marketing gimmick. Use something with fluoride.
Use a tongue scraper (metal one). It prevents bad breath. A spoon will not work well.
Use beard oil if the skin underneath is dry.
Use shampoo without perfume if you have sensitive skin.
Clean your ass in the shower. Seriously, focus the beam on it and put your fingers in the crack.
Switch pillow sheets often if troubles with acne (or put a clean towel on top of it).
Do not shampoo too much. This depends on hair type. Are you oily or not? Is your hair thick or thin? Take note of this. Experiment between 2x/week to daily shampooing.
Switch underwear once per day.
Wipe until there's nothing left.
Always travel with a nail clipper.
Alternatively, travel with some rags. You can go surprisingly long just by cleaning face, armpits, and nether regions every so often.

Health

Visit a dentist once per year.
Visit a doctor for yearly checkups.
Wear ear protection when things are loud! (concerts, cinemas, power tools, etc.) Seriously, I have tinnitus. You do NOT want this! Trust me.
Be aware of your posture. Do stretches, cardio, and strength exercises. This doesn't mean you have to tense your body into a correct posture, just make sure you do not sit in an awkward position for extended periods of time.
When doing cardio, you can check your heart rate using a pulse oximeter to keep around ~150 BPM. This is about the lactic acid treshold for me. You can keep going at this level for a long time. Without this, people tire themselves out in cardio due to pushing too hard. They then lose the motivation to continue.
When doing strength training, be very observant of your own form, so that it is good. Otherwise you can injure yourself.
If you take any drugs/medication, always check for interactions with each other. They can be dangerous. This is called a contraindication.
Ensure your living environment is not too dry. It can cause disease. A living area should have an absolute humidity of around 7-8.5 grams/m³, which is a relative humidity of approximately 40 to 50% at 20℃
Ensure that your living environment also is not too humid (above 70% relative humidity at 20℃), as it can promote the growth of microorganisms.
Do not use ultrasonic humidifiers, they spread droplets in the air. If your water contains impurities these can affect lung health. Use evaporative humidifiers instead with filters kept clean.

Cleaning

Never mix chemicals without checking if it is safe to do so. Some mixtures can produce toxic gases.
Soap is a household universal solvent. Anything grimey can be soaped away.
Acids are good at dissolving limescale. Dilute (5-7%) vinegar is relatively safe to use for this purpose.
Bases are good at dissolving organic matter. Unblock a sink with sodium hydroxide.
Mixing acids and bases does not yield much useful and can be dangerous. If you mix sodium hydroxide and hydrochloric acid you get table salt, water, and heat.
Bleach (Sodium hypochlorite) is useful for whitening fabrics.
Ammonium hydroxide is good for cleaning windows/mirrors and leaving them streak free. Use a dilute solution.
Use steel wool to clean surfaces that are equal or harder than steel (ceramics, glass, steel). It is the easiest way to clean really hard surfaces. Do NOT use it on plastics (plastics are softer than steel). Do not use on delicate hard surfaces like a knife's edge.
Use steel wool to clean cloudy glassware. Glass is harder than steel, so you won't scratch it.
Do NOT use hard materials (like forks and knives of steel) on teflon-covered pans/surfaces, it can damage the teflon.
It's okay to use comfortably warm water when hand-washing dishes. You don't need it to be scalding. See this video.
Put coarse table salt into your dishwasher! This prevents lime buildup on your dishes. It exchanges for instance calcium carbonate with sodium carbonate, which is far more soluble. Your dishwasher should have a big knob you can unscrew which is filled with water. The knob should say "Salt". Put salt in here. This salt needs to be replenished periodically.
Add pre-wash detergent to your dishwasher. Even if it doesn't have a pre-wash detergent slot.
You generally do not need to rinse kitchenware before putting them in the dishwasher. Same video as above.
Get a robot vacuum cleaner, it's so much more convenient than a normal vacuum. There's no honor in toiling.

Clothing/Fabrics

Read the instructions on your washing powders/liquid for clothes.
Find out the hardness of your water to dose correct amounts of washing liquid or powder.
Do not leave clothes in the washing for more than a few hours. It can get moldy. Set a timer.
If you hate ironing (I do), just avoid garments that need it.
Store linens (bedsheets) in open air. This prevents them from becoming smelly (I've tried baking soda, it doesn't work, the only way is to hang then with good ventilation.)
Make your wardrobe layerable for maximum reuse.
Be aware of skin tone when choosing clothing colors.
Buy clothes that fit, or taylor them. Fit is key to looking good.
Shoes can be deodorized by spraying very dilute (~0.5%) vinegar inside and on the shoe. Thoroughly dry afterwards.

Mastery of environment

Learn the concept of relative humidity. Different temperature air contains a different maximum amount of water (less with lower temperatures). This is why condensation occurs when the air temperature goes down.
Learn how rust works and how to protect against it.
Learn why wood rots and how to prevent it.
Grasp the concept of power and energy. Power (Watt) is energy per second (Watt = Joule/second). A joule is a unit of energy. This prevents you from getting tricked by scammers and charlatans.
Know of Ohm's Law, and amperage ratings of electrical circuits. This can prevent fires.
Be aware of specific heat capacity, and enthalpy of vaporization. Especially that of water: Water requires astonishing amounts of energy (Joules) to change its temperature compared to many other substances.
Consider alternating and direct current circuits; do not wrap a high amperage alternating current circuit into a coil. This creates an inductor. It can also create eddy currents. This can have dangerous side effects such as heating a metal.
While cooking, do not put water on an oil fire. Put the lid on instead.
Conventional fires need three things: oxygen, fuel, and high temperature. Remove any to kill the fire. Water mainly removes the high temperature.

Personal

Try things you think you won't like every once in a while. As long as that thing doesn't leave permanent changes. E.g. try a food you never liked.
Try things your friends will think you will like from time to time, within reason, of course.
Avoid people that make you feel bad in an unconstructive way.
Do not fear or avoid bad news that is actionable.
Be aware of your gut instincts and heed those signals.
Avoid idolizing. All will eventually disappoint you.