what is devising theatre and what does it involve?

Devised theatre is a form of theatre where the script originates not from a writer or writers, but from collaborative, usually improvisatory, work by a group of people. 
1-begin by focusing on form, and then extract thematic ideas and work with them retrospectively.
2-use games as a starting point.

3-Be passionate about your source material

4-research

5-Get the material out there as soon as possible

6-Keep an open mind

7- Everyone works differently so get introduced to new ideas and new ways of working this out.