I have designed and assembled a PCB. All connections are fine and leds are blinking when i open the CUBEMX. But I don't know what to do after this point. Do I need to put some drivers before using DFU? If so, i don't have an ST-LINK Programmer how can i use the only input for this pcb (which is PB11,PB12 as AN2662 suggests). Its also interesting that device is responding to CubeMX . Does device has any code in it?
MCU : STM32F405RGT6

Your question is not clear. But i will try to answer to your question with my understanding from the question. First thing that you need to find the datasheet of mcu. Now, you can able to find related pins for programming. Using these pins, you can program your hw.
if you want to put dfu mode to your hw, Please check "Application Note 2606 STM32 microcontroller system memory boot mode".
Also, i wrote post about system memory bootloader, here is the link: https://engineeringvolkan.wordpress.com/2020/05/08/1928/