Start by thinking of the people you are addressing, as well as the person you are describing: the eulogy is about the person, but for the audience.
Who are they – family and close friends only or others too? There may be specific things to say or avoid.
How will they feel? Listening to you will obviously be highly emotional for those closest to the person, and some people will be in tears. But this doesn’t mean the eulogy should be mournful and depressing. People will be grateful if what you say is uplifting and inspiring.
What do they want to hear? Most people want to hear good things about a person who has died, and forget the bad things. But people don’t become saints just because they die. Your audience will want to feel you have captured the essence of the person – what makes them special. So be honest, but selective.
How long should it be? Even in the circumstances of a funeral, many people find it difficult to listen to one person talking for a long time, so a eulogy should really be over in a matter of minutes – just how many is a matter of individual choice.