How to get started editing the source for Chromium?
The Chromium.org site has some resources :
- General http://www.chromium.org/Home
- Building on Ubuntu : http://code.google.com/p/chromium/wiki/LinuxBuildInstructions
- Conventions : http://www.chromium.org/developers/coding-style
- Contributing : http://www.chromium.org/developers/contributing-code
(Sounds RTFM-y, I know, sorry)
That said, you're probably better of tweaking an existing browser codebase rather than creating one "from scratch" - a task which I would assume to be daunting at least.