1) Install WinUAE
2) Copy the Kickstart files into the WinUAE folder and make sure that WinUAE finds them
3) Install GameBase
4) Install GameBase Amiga into the GameBase folder
5) Extract Extras, Games, Music and Screenshots into the GameBase Amiga folder
6) Run GameBase Amiga and use the Tools -> Verify available files option.
My thanks go to all who helped in creating the GameBase Amiga/MAGE project, in particular
Belgarath and eLowar.