Some of the best known books written as if they were a diary are "The Diary of a Boy in Distress", "Ally's World" and "Dear Silly Diary". Write a best seller about your daily life like a diary!
Steps

Step 1. Decide on the topic
Will it be the life of a boy or a girl, a teenager or a child, or an adult or an old man? If not, will it be about yourself? Decide where he lives and what his characteristics are, such as his name and hobbies. Think about their likes and dislikes. Spend a great deal of time thinking about it.

Step 2. Make up other characters
It is certain that family and friends are the most important thing in that person's life. Also, these characters are likely to do very interesting things in the plot. Decide what people will be around him, such as his parents, friends, other family members, and enemies.

Step 3. Come up with a simple plot
If you just want it to be about his life, that's fine. However, you may run out of topics and the story gets boring. Think about what your character's life revolves around, such as his pet, a relative, his qualifications, his friends, among others. Once done, come up with a story that you can tell about it. Give more thought to the ideas that come to mind and then write them down (remember that you can change them along the way).

Step 4. Decide on the title and cover
You can draw it or take a picture. Remember that this is a journal, so try to make it as a cartoon or something that relates to the life of that character. You can also make it look like a newspaper cover about something. Then put illustrations in the book if necessary.

Step 5. Start writing
Don't forget to label the days and make each entry about the length of a page. Also decide how long you want your journal to last, whether it's a year, a decade, or until you run out of pages.
Consider the format of the book. There should be no chapter or anything like that in it since it is a diary. Try to have one to see how a journal really works

Step 6. Get it published
Try reading the article How to Publish a Book for more information.
Advice
- Read some newspaper-like books to get an idea. "Diary of a Boy in Distress", "Ally's World" and "Dear Silly Diary" are good books. Just don't copy them.
- Don't make it an exact copy of another book. Be original!
- Optionally, enlist the help of a friend if it's more fun this way.
- Add images for a more vivid appearance.