Our parents purchased a set of World Books when I was a girl. They lived in my bedroom and I found them wonderful for just looking at pictures and learning facts. Books remain a default resource for me: stitch dictionaries, pattern drafting books, books on designers and art.
Non-designers are always curious about where ideas come from. Does it come from the yarn, from something I see, or where? It is a bit of a chicken and egg thing. A great idea without the right yarn is just an idea, and a great yarn without the right design is just yarn. So it is a convergence of the right idea and the right yarn, combined with stitch pattern and details that make it all work. Like all things creative, it is probably 10% serendipity and 90% hard work.
The first time someone seriously asked me this question was when I realized I was really a knitwear designer. I was capable of pulling all the elements together to create the right design. I didn't start out designing knits, so when I decided that was what I would do it required a fair amount of education to be able to do it. All of the skills I had learned using paper and fabric to create garments still applied, but with knits you create the fabric and the shape and do the construction, so I had to learn how to create the fabric I wanted, and then how to fashion shape into that fabric, and then how to join shapes together to make my ideas come to life. Then I had to learn how to write a pattern so someone who wasn't sitting in my head could create my idea.
Like most hand knit designers, I started out by knitting and trying out different things and learning as I went along. I took classes, read books, and did a lot of experimenting. But I can't produce enough if I do everything as-I-go, so I had to learn a different way. I do a combination of pattern drafting and knitting swatches to create most patterns. I rely on someone else to knit it and give me feedback as they go so we can change things that aren't working. The internet, of course, makes all of this possible!