Meditate a LOT. I cannot stress this enough.
When you are ready:
Start by laying back and closing your eyes. Take deep breaths and clear your mind. Ground yourself. My personal grounding method is imagining roots growing up from my core into the heavens. Then from my core, roots digging to the center of the earth.
Then the rest is not very easy to explain.
Its different for everyone but here is my UPG:
I usually start by meditating and concentrating on blackness. Then when i feel its time, I imagine myself standing in the emptiness. I call out the name of who im channeling. It takes a while but they usually appear as HUGE and MASSIVE figures in the blackness. Their voices booming.
Its scary at first and only do it when you are sure you have the energy. You get the hang of it. Good luck! :D
PS and UPG the scenery usually changes from black to whatever the gods change it to.