A detailed description of the focus groups can be found in Detmar et al. (2006). In the manual there is a section on this in chapter 2 p. 25/26.
Detmar, S. B., Bruil, J., Ravens-Sieberer, U., Gosch, A., Bisegger, C., & European KIDSCREEN Group. (2006). The use of focus groups in the development of
the KIDSCREEN HRQL questionnaire. Quality of Life Research, 15, 1345-1353.