I'm going to be blunt so please forgive me in advance. It's obviously very infected and you aren't up to the job of wound care since it's progressed to this state. I suggest you euthanize this chicken.
To care for a wound like this would require surgery to remove all the infected tissue. That's all the black scabby stuff and all the pussy material underneath. It's called debriding. It would be a daily affair of cleaning and debriding and dressing with antibiotic ointment. The chicken would need to go through a round of antibiotics on top of it and be kept in a clean environment. A vet would charge you a small fortune. To do it yourself would require a big commitment of time and energy. The whole process could take up to two months.
Even if you felt up to the challenge, you may still end up losing the chicken to infection. This is why I suggest euthanizing the chicken.
