I am starting to build my first multilayer board and wish to know how to create a flash file, or how can I extract a flash file pattern from an existing board .brd file to my library.
You can export from an existing brd file by using File, Export, Libraries - this will create padstacks, dra files etc. for the elements inside the brd file.
If you wan't to create a new flash symbol just select File, New and set the type to Flash symbol. You could use Add, Flash when inside the flash symbol to create the actual copper for the flash symbol