from scratch

Từ điển Oxford Advanced Learner 8th

from ˈscratch idiom
1. without any previous preparation or knowledge
•I learned German from scratch in six months.
•He built the orchestra up from scratch.
2. from the very beginning, not using any of the work done earlier
•They decided to dismantle the machine and start again from scratch.
Main entry: ↑scratch_idiom