Effectively writing the conclusion to a biography is not difficult. A manuscript conclusion highlights the key points in a person's life.
You should have your notes organised. If these are not organised, compile the manuscripts into chronological order.
Extract from these notes, and the biography, the high and low points of the person's life. Jot these down into an outline. This will form the frame of your conclusion. A pertinent question to ask is, How did these events mould this individual? What was the subject's reaction to the situations and how did this person in turn shape events? Also ask, Who were the key people in the person's life, and how did they shape this person?
Write the observations that you have considered from the questions you have posed. Frame this in terms of the person's long-term legacy or ongoing impact in the world.
Get feedback from those who have known the person, if there are any to be found. If this is a biography for a living person, ask that person for clarification and insight about the manuscript. A historical biography will necessitate further archival research.
Now it is time to synthesise your questions into a conclusion. You will be writing on these key points. Who were the primary people who influenced this person and the people whom this individual influenced? How did this person change the world, and how did events shape this person? What is the legacy of this person's life?