The secret sauce
Unfortunately, there's no magic bullet or secret formula people are using to write every day. You won't find Michael Jordan's Secret Stuff of journaling (did you catch that Space Jam reference?).
They just write.
Here's the simple answer for you: Just start.
Copy the veterans, the people writing every day
That might not be the answer you're looking for, but that's what you must do.
If you want to start keeping a journal, do what journal veterans are doing. And that's write. Write about whatever comes to mind.
If anger comes out, write it down. If you're sad, write that down too.
Don't overcomplicate things
You're making it too complicated.
You're not George R. R. Martin writing the next book for A Song of Ice and Fire.
This isn't a criticism of your thoughts and feelings either.
By simply starting and showing up every day, you give yourself a chance to be consistent. That's how you learn and build a habit of writing every single day.
That's the secret sauce.
A journal isn't meant to be produced for live television. Nobody is watching over your shoulder to tell you your writing stinks.
Your journal doesn't have to make sense
The best part is your writing doesn't even have to make sense.
Your daily writing can ramble. It can be completely incomprehendable or full of jibberish.
You can use slang words. Make up words. If you don't feel like using punctuation or paragraphs, don't.
Even if you hate what you write about every day, that's OK.
Over time and with consistency, your writing will get better.
You'll get all of the crap out of your head and start making sense.
Practice writing. If you only feel like writing a few words or a couple sentences, do that. Write a few pages if the words are rolling off your tongue.
It's practice. That's all it is.
Find extra motivation
If you're still looking for extra ways to be consistent, challenge yourself. First, try to write three days in a row. If you can do that, try seven days in a row next.
Use an app to give you writing challenges to help. Apps can track your daily writing and reward you when you complete certain tasks.