HELP! Installing firmware/bootloader to ATmega8u2 and ATmega2560

Hi,

Hi,
(sorry for my english)
I created the DPS, which has ATmega8u2 processors and ATmega2560. I need to know the progress of the work, how do I install the necessary software to them. ATmega8u2 has to work as a USB serial converter.
I know I have to upload the bootloader ATmega2560-au but do not know how.
I know that I upload firmware to ATmega8u2-au but do not know how. It is necessary to install the first bootloader ATmega8u2 or can I just install the firmware?

Please help me write the procedure and what to do.
thank you