I self-studied Group I without any external assistance.
I scored an 80 in Cost Accounting and Financial Management, so I'm fairly confident my advice would be an experienced adviced.
I referred to P.C. Tulsian, Padhuka and the Scanner.
Start studying each chapter by P.C. Tulsian. Conceptually, I believe it's one of the best books out in the market. Once you're done with each chapter in P.C. Tulsian, study Padhuka. And bear in mind, follow this sequence chapter-wise. And finally, when you believe you have your concepts straigthened out, do the questions in the Scanner. If you're able to complete the questions in the scanner properly, you're fit to go.
But don't dishearten yourself if you don't get the questions in the Scanner right. It happens a lot. The important thing is you learn your mistakes. A lot of the questions in the past papers require a high amount of conceptual intellect. I got many of the questions wrong, but after I read through them, I realized my mistakes were very minor. Conceptually, I was doing very well.
So well, that's my advice. Hopefully that'd help you out.
Good luck for your examinations. And remember, everything in the question paper can be found in the books. So revise well.
Regards,
Sam