I have previously written about Android Design and how too many people approach me with iOS-looking designs for what will be an Android App. I previously created an article outlining some tips for porting iOS to Android. Assuming you now have an Android-esque design, what about variation of the UI on differing devices and for orientation switch?
There are broadly two types of project - what I call "cost sensitive" and "user experience sensitive" projects. Cost sensitive projects tend to leave fragmentation of the UI to the developer and there’s some great information on the Android Developer site.
User experience sensitive projects need to define more upfront. Designers need concise guidelines. The problem is that, depending on where you look, Google’s documentation appears either too technical or too verbose and contains no pragmatic tips to shortcut some of the possibilities. In short, the documentation seems more for developers than for designers.
One of my followers on Google+, Maksim Golivkin, has pointed me to a SlideShare presentation that he has created that provides a useful fast-track for designers. It defines what’s needed and contains some great tips to cope with screen variety.