To start from scratch

Idiom: To start from scratch - To start something from the very beginning


Context #1
Jen:  I really admire Keisha. She has become a successful restaurant owner in just three years.
Kat: I agree. And did you know that she started from scratch? There were no restaurants in that area and she raised the startup money herself! She even cooked the food on the menu at the beginning until she had more money to hire a cook.
Jen: That’s really impressive! Nowadays, it’s so hard to be successful when starting from scratch. Usually, it’s much easier to just take over a business that is already established.

Context #2
Jack: What’s wrong? You look really sad.
Terri: Oh, I’m just having problems finishing my new book.
Jack: Really? You usually put out a new book every 6 months or so.
Terri: Well, not this time. I had finished about half of the book when my editor told me it was all wrong and didn’t make sense. He told me that I would have to start over again.
Jack: Wow! You had to start from scratch after finishing almost half of the book? That’s tough.
Terri: I know. Hopefully, I can do it a little faster this time.

Meaning: “To start from scratch” means to start something from the very beginning without using anything else as a starting point. 
In context #1, Keisha started her business from scratch, meaning that she started her business new. 
In context #2, Terri had to throw out half of the book she had written and start the book over again from the beginning.