What are the Stages of Negotiation? And what is the most important stage?

It starts with preparation for the negotiation which includes planning, meeting with stake-holders who will be to prepare for the negotiation.

Stating the rules and what is to be negotiated, putting the top targets to be achieved, timeframes, calling the other party to negotiate,

Prepare the room for negotiation if it is at your place and then you play the game of negotiation.  It is your yard and you have to play well.

The most important is the one where you place the rules and explain it to everybody.