Hi, first you must create an empty .TAP tape file with your emulator, insert it in the virtual tape recorder (some emulators like Spectaculator do this automatically) and set it to "record". Then you should save the game as normal. Check the virtual tape recorder - if you did everything correctly, the saved game will appear in the virtual tape file.
Emulators have different procedures for doing this. Which emulator are you using, actually?