In order to make a habit, I makeam making a rule which is about 21 days, 100 days, 365 days and 600 days.

Honestly, simple present is one of the few tenses that doesn't work very well here. It most strongly suggests that this is a habitual action - you make this rule regularly, repeatedly, whenever you feel the need to form a habit. "In order to get to the shop, I walk down the street".

You could use simple past, "I made", or present progressive "I am making", or present perfect "I have made", or simple future "I will make", or even future progressive "I will be making". In this context the differences between them are fairly subtle and I won't explain unless you want me to, but I think they all work a great deal better than simple present. "I am making" seems most like what I would say.

I would consider separating the "which is about" clause somehow. "I am making a rule, which is about" or "I am making a rule, a rule which is about", or "I am making a rule. It will be about", something like that. But I think what you have is okay.

The meaning of the days that I suggested is that they are goals of days in a row.

Awkward to get this to work.

More natural: "The numbers signify goals of days in a row."

According to a book about brain science, it is necessary to do 21 days in a row of making a routine in my cerebellumin order to get my cerebellum used to the routine.

Slightly guessing at your meaning here. What you wrote says that you will "make a routine in your cerebellum" for 21 days in a row.

With the aid of my cerebellum, I don't need to concentrate less to do a thing.

Or "I don't need to concentrate as much".

A note: these five sentences work in simple present because they feel like you are narrating a story. But they would probably be more natural in a future tense: "I will need to concentrate less/I won't need to concentrate as much". These things haven't happened yet, after all.

Or "I should need to concentrate less".

After 100 days in a row, my subconscious remembers the thing.

(will remember/should remember)

If I don't do the thing, I feel uncomfortable after 100 days.

The order you have this in suggests that 100 days after you don't do the thing, you feel uncomfortable. You miss a journal in January, and begin to regret it in April. Not very useful. Dropping the 100 days from the end fixes this, and we remember it from the previous sentence so we don't need to repeat it for context.

But if you did want to repeat it: "After 100 days, if I don't do the thing, I feel uncomfortable" works.

(I will feel uncomfortable/I should feel uncomfortable)

OIn one year, I can feel a great sense of achievement in my work.

(I will feel/I should feel/I will be able to feel/I should be able to feel)

Afternd 600 days later, I don't need to think about it because the thing becomes mya habit.

"after" and "later" do the same thing here, so one is unnecessary. I replace it with "and" because that gives a sense of moving on/finality. "And this is the last thing I am saying".

(I won't need to think about it because it will have become a habit/I shouldn't need to think about it because it should've become a habit)

I have started to write a journal in English, and today imakes three days in a row.

Or "today is the third day in a row".

Again simple present doesn't work well here.

I hope, I can makedo this writing duringfor 600 days or more.

That's the closest to your original sentence I can get.

More natural: "I hope I can keep this up for 600 days or more", "I hope I can do it for 600 days or more", "I hope I can keep writing for 600 days or more".