At Scottish D&D it is done by 3 staff telephoning 3 differrent coastguard stations (or some airfields), asking them to switch the DF equipement from "marine VHF" to "Air", then getting you to transmit whilst they get a bearing. You may have been asked to wait for a few moments whilst the calls were made, and then make a long transmission - normally a count to 5.
These 3 bearings are then manually entered in to a PC program which plots your location on a map and displays the "area of uncertanty" triangle. The program will also give a range and bearing from your approximate position to one of many listed sites.
The plotting can obviously done manually as well.
London D&D have Auto-plotting.
Simple really.