Whenever I had an idea for a craft, I would quickly sketch it on a scrap piece of paper. What I ended up with is a huge pile of loose papers that I would shoved into craft magazines.
The problem with this is that I'd have pieces of paper everywhere so when I wanted to find a particular design I'd have to search in all the different magazines. Then, of course, there's also the problem of the papers falling out of the magazines. It was very disorganised!
I also coloured in the designs to add interest and to give the details of what it'll actually look like.
If you're thinking of starting a visual diary then you'd need a notebook. How big really depends on what you want. I used a medium size hard cover spiral bound book but you could also carry a mini notebook to jot down any ideas quickly. Then when you get home you can trasfer it to your usual book.
Anything can go into your visual diary. You can record ideas, thoughts, feelings and observations. You can even stick on its pages some fabric samples and a list of things to make with it. The possibilities are endless.