The Helsinki Design Lab (an entity within Sitra, the Finnish Innovation Fund) has created a Design Ethnography Field Guide which is a booklet that participants of the HDL Studios use when venturing into the field to see the reality of a system as it is lived and experienced on the ground. It is intended to be the minimal starting point for this kind of activity. They supplement this document with group discussions to prepare participants and adjust the booklet as needed in different situations.
The download page also contains some other resources that may be a useful starting point.
And obviously more resources can be found on this very blog.