You Just. When you are floundering with a task or project, someone will pop up and give you the key to solving the problem. The bits left out in the manual. -- Knitnan
Every instruction set assumes. Even the most detailed of instruction sets assumes the reader knows what the tools are, the techniques mentioned, and what the key points mean. One crafter's definition of "the cracking point" is different from another's. One is good, another is bad.
There's a certain number of hurdles associated with learning the finer points of skills that have passed into the realm of special interest. The process requires passing information on from those with more experience to those with less.
When those skills evaporate entirely, the re-learning curve is extreme. People who, in the past, have taken progress pictures are invaluable to those attempting to recreate the skills involved.