The answer to this question can be found in Chapters 5 and 6. The basic answer is that most hunter-gatherers got agriculture through diffusion while those who invented it on their own did so gradually.
Diamond tells us in Table 5.1 that only five areas in the world saw people develop agriculture on their own. This means that almost all hunter-gatherers in the world became farmers by adopting the farming technologies and techniques from other people.
For those who did invent farming, Chapter 6 tells us that they did so in a step-by-step fashion. These people started to produce food by planting seeds from a few beneficial plants that they could later gather from. Then they might have started to plant even more. Eventually, they might have settled down near to the places where the crops were planted. In other words, they did not simply stop hunting and gathering one day and decide to go straight from a pure hunter-gatherer way of life to a pure agricultural one.