Open with a strong image. When attaching an emotion to an idea, it is important to make that emotion strongly felt from the very beginning. For example, if you are writing about the plight of children in poor countries, you may want to start by describing the sort of circumstances that the child lives with every day.
Maintain the same level of emotional intensity when logic is introduced in to the argument. While the introduction and conclusion are primarily emotional, the bulk of the speech or written piece should be based on facts. However, to keep the emotional attached, you can comment on the facts as they fit your needs. For example, after mentioning that a high number of children live in poverty, you can say something like "this is unacceptable."
End strong. The conclusion is especially important in a pathos piece because this is where the strongest emotion should be felt; it ideally should stir the audience to action. For example, after talking about the plight of poor children -- perhaps repeating the same image from the introduction -- you can tell the audience that is it their responsibility to help their fellow human beings.