How do i make an EMP gun or a HERF gun?

i saw somthing on google about a cool gun that you can point at a light bulb and it will illuminate it. i was wondering how to make such a gun so could sombody out there tell me how? i already know its made out of a microvae thinger and a horn antenna but i dont know what part of the microwave is needed and how to make the ideal horn antenna. pleese help. best wishes