Краткое объяснение значения и использования английской идиомы "Start from scratch".
Meaning
To start from scratch is to begin from the beginning, to set out on some action or process without any prior preparation, knowledge or advantage.
Scratch has been known since the middle of the eighteenth century as a sporting term for a line scratched on the ground that acted as a boundary line or starting point. The first example in the Oxford English Dictionary actually relates to cricket and indicated the crease, the line drawn in front of the stumps where the batsman stands. But the term is much better known from boxing, or rather from bare-knuckle fighting, in reference to the line drawn across the ring to which the boxers are brought to begin their bout. This gave rise to expressions like to be up to scratch, to meet the required standard in something.
Your phrase appeared a century later, by which time scratch had also came to mean the starting line for a race. Competitors who began from this line had the least favourable handicap and so were given no advantage. To start from scratch meant you had been allowed no odds in your favour. It has been generalised from that.