Once a typically developing  infant acquired good control of trunk extension, balance and UE support in prone, they will usually start to push up into prone kneeling and start to explore different ways of moving forwards. Eventually, with time and practice the infant will acquire  a mature reciprocal crawling pattern and the ability to move around at speed. 

TD infants also learn to support themselves in upright kneeling, crawling up onto and off a step, up and down slopes and over soft surfaces

In this set of videos you can review the Will/'s the development of Will's crawling abilities, starting with an overview of learning to crawling from 6-12 months.