If you're a non-developer looking to get started with building Flex Viewer widgets, you might find the following document useful:
When I started at Esri UK, I had a fair amount of development experience but I was completely new to Flex. I ended up taking a lot of notes while I was learning my way around the Viewer, which were eventually turned into a beginner's guide aimed at non-developers.
The guide is a high-level overview of the widget development process and the various technologies involved. For more detailed step-by-step instructions, ArcGIS.com has lots of helpful resources.
Any comments and suggestions are welcome, and we really hope you find it useful!