http://flows.panarchy.com/index.php?title=Development_HowTo

Basically:

1. Pick a programming language: python, php, etc.
2. Download the template for components in that language, or in another language if your choice is not available.

http://flows.panarchy.com/sandbox/templates/

Mercurial Repository:
http://code.google.com/p/flows-dev/

3. Add in the desired functionality
4. Make your component accessible via HTTP

that's it. srsly.

Q: But why is it so simple?