Speak to the subject of the biography, if this is possible. Set out some clear guidelines, so neither of you will be disappointed with the resulting biography. Ask the subject about his early life. You will need to meet him several times, so you can build up a picture of him. But remember that this is just one version; you will also need to do independent research and speak to people who are close to him. You may also need to speak to some who will not be so flattering.
Speak to people who are, or have been, significant to the subject. You will need to approach these sources in a professional manner, by contacting them, asking for help and for an appointment. Take careful notes when talking to your sources, being careful not to misquote anybody. You may choose to speak to friends, family, colleagues, political allies and maybe even political opponents.
Research key campaign speeches and interviews. Build up a chronological picture of the subject, noting any important moments or turning points in his political life. Speak to campaign managers and people who helped him on the way. Research past copies of newspapers. Read past interviews, but be careful not to use a "cut and paste approach." Listen to speeches and watch recordings, as this varied approach will help you form a more rounded view of your subject.
Structure your biography in a logical way. This may vary, according to your own preference. Some biographers adopt a strictly chronological approach; others prefer to interweave past information with current developments. Add color and individuality to your narrative. The biographer's voice may not be at the forefront, but it will be present.