Best way is just OPEN file$ FOR BINARY:
SUB CopyFile (source$, dest$)
OPEN source$ FOR BINARY AS #1
OPEN dest$ FOR BINARY AS #2
temp$ = SPACE$(LOF(1))
GET #1, 1, temp$
PUT #2, 1, temp$
CLOSE
END SUB
For a professional app, you'll probably want to do some basic error checking for _FILEEXISTS and _DIREXISTS, but that's the basic process to quickly copy a file.