How it works
A big program is built from small parts, the way a jigsaw puzzle is built a piece at a time.
Make each part work alone first, and then join them, one at a time, checking after every join.
Give every box a clear name. Two boxes with the same name clash and share one number.
Reuse a part instead of copying it. A custom block is written once and used anywhere, so changing it once changes every use.