Sit in easy pose, on either wool or cotton padding of some sort. Some people use sheepskin, others prefer a folded blanket. Your foundation should be solid so you can sit still. You can chant sitting in a chair or even lying in bed if your physical condition requires it. Be sure to keep your spine as straight as you can.

He who masters mantra, masters God Himself. Perfection can't be expected to be easily achieved. You will have to be the best judge of what works best for you. One of the many advantages to group sadhana is that you can just surrender to the process, as someone leads the chanting (or, you can lead it and do it "your way").