Activities (for each type of screen)
- define high-level screen elements
- like body, navigation bars, tool-bars,
- settings, place for advertising and cross-references,
- place screen elements consistently
- detail high level elements
At this stage of the project you have to be careful with proportions of used screen elements and with pixels.
Remember, the more screens you can detail, specify and deliver/communicate to a development team, the less surprises you will have, when those screens are implemented.