Consensus building involves finding the middle ground when difficult issues are being debated and then bringing in more members to support that compromise position so that a resolution has a maximum level of support from member states.
During the Cold War, the UN was very divided and it was difficult for resolutions to pass with more than 60-70% support of the members. Following the end of the Cold War, the UN has increasingly tried to work toward consensus, where many resolutions are adopted unanimously by all voting members. Even without the divisions of the Cold War, this is still difficult to achieve. Delegates must engage in extensive dialogue with each other regarding the specific interests of their countries and their preferences regarding the language in the draft resolutions under discussion.
It takes a particularly skilled diplomat to identify a solution that is agreeable to a core set of delegates and then bring others into that group. It often involves a degree of compromise. Delegates should not only seek to clearly articulate their own preferences, they should also listen carefully to other delegates to see where their interests overlap, and where there might be room for compromise.