In this paper a detailed description of how to simulate charge
transfer processes in a gaseous device is presented, taking the
Gas Electron Multiplier (GEM) as an example. A 3-dimensional
simulation of the electric field and avalanche is
performed. Results on charge transport are compared to
experiment and agree within experimental errors; avalanche
mechanism and positive ion feedback are studied. The procedures
used in the simulation are described in detail, and program
scripts are appended.