Working directly in the medium
I'm in the middle of building the largest and most complex piece of software I've ever worked on. It's a stepping-stone tool for high schoolers and undergraduates to start reading scientific literature.
The basic features are all in my head. As I code, I build whatever gives me the most bang for my buck. So far, I've been able to create a pretty sophisticated working prototype without a to-do list or any other detailed plan.
Very often, I have to stop and skill up before I can build a feature. I might just read an article or post on Stack Overflow. For bigger learning projects, I take a Coursera course on some concept I'm unfamiliar with. I just plow through it and keep getting things done.
When you're working directly in your medium, the project itself tells you what to do next. It reveals itself step by step, moment by moment.
It's the same when I'm teaching music. I never plan lessons. The right thing to do reveals itself to me from moment to moment.
Useful planning and meta-level thinking
Useful planning is generally obvious. Make a list before you go grocery shopping. Talk out what your needs and desires are when house-buying with your spouse. Check the personality traits of the dog breeds you're considering. You have an obvious question, you look up the answer, and you move on.
Try before you buy. Do some independent research before you apply to a PhD program. Hang out a lot before you have sex. Take the car for a test drive.
It's the finger pointing at the moon, not the moon itself.
It's a long to-do list that doesn't translate into action. A spreadsheet where you gather information in order to forget about it. A long chain of thought culminating in an epiphany that goes nowhere. An argument about an issue that you never work on directly.
Bad planning like a belief in telepathy. It makes you feel like your private thoughts can change the world. The quintessential example? A college humanities essay that gets read by the student, the professor, and nobody else, but which the student remains proud of for the rest of their life.
The mechanism is the mission
If you have a big dream, set yourself up to work directly in a medium that would help you realize it. If you want to build technology, learn to code, build electronics, machine metal. If you want to do work in the life sciences, set up a little home lab. If you want to spread an idea, create a website, write articles, do a podcast, organize a protest, make a T-shirt or a bumper sticker, and start talking about it with everybody who will listen.
All this work will let you create mechanisms. Mechanisms work in a repeatable way. They're separate from you. They're interactive. They change the world.
Don't worry so much about making a useful mechanism. Don't worry if you can't build your dream yet. Just let yourself discover the pleasure of manipulating the medium, of building mechanisms. Get in the flow. Learn what it feels like to be making.
Be obsessed with creating visible signs of progress. Things that work. Minor accomplishments. Change the world a little bit.
Spend enough time building mechanisms in a certain medium, and you'll be shocked at what you can create. And the changes you can make will grow.