To start something from the very beginning. To begin something with nothing

Francesco knows absolutely nothing about computers, he has to start from scratch to learn how to use it

Francesco non sa assolutamente niente di computer, deve partire da zero per imparare ad usarlo

If I want to eat a good cake, I have to do it myself, starting from scratch

Se voglio mangiare una buona torta devo farmela da me, partendo da zero

In racing the “scratch” is the starting line, in old times literally scratched in the dirt. Horses or people racing with a handicap, start ahead of the line; the ones who don't need an advantage “start from scratch”, i.e. from the very beginning