The best way to plan what type of B-Roll to get for your project is based on the topic itself. Essentially you want to get the largest variety you can get. If the events you are covering are current, then you can try and capture footage as it is happening, but for most home documentary projects this is not the case. The best way to plan is to know what types of footage is available. There are photos, stock news and other types of video footage, location shooting, and many others. When selecting what types of footage you want you will need to plot out exactly what each interview clip is talking about and try to pair it with footage that will illustrate their discussion. If you are trying to do a digital video project about a historical event you may only be able to get photographs, so you should know exactly what your limits are going to be. If someone is discussing a certain event you can shoot video of the locations where it happened, show historic photos of the event, and if some related event is happening you can record it and use that to illustrate it.