I always have a preparation before healing someone or giving or receiving an attunement. Because each person is different, they may do different things to prepare. I always make sure that I am grounded, focussed on the matter at hand and I ask my guides and theirs that they receive nothing from me nor I from them. That I feel is very important. When I took my Karuna Reiki Master classes, my teacher didn't do this before she passed me my first attunement so my energy was thrown off and I was sick because I got her baggage. That is always my rule number one is that I don't give them anything that I have and that I pick up nothing from them.
A "channel to be open" would be the crown chakras as well as your hands. For Usui my crown chakra was opened and the symbols placed there as well as in my hands. For Karuna, my crown chakra was opened and symbols placed there, as well as my heart chakra open and symbols placed into my hands.
As far as light coming out of my hands, I don't see auras so I really have no idea. I imagine that if a person saw auras, they may see mine change when dealing with reiki or any energy work. But the most common that I have been told is either heat or tingling are felt from my hands or when I send by distant. But each person is different and receives differently each time.