If you are getting ready to film a sunset you have to make sure that you are in correct position, not just to see the sun but the other objects that you want to include along with the sunset. These can be things like the coastline, the skyline of a city, or a mountain top. The sunset may highlight this, shadow it in darkness, or otherwise prepare the correct framing for this. You may want to check out this area of the sunset ahead of time so that you have a general idea of what you want and can have in the image.
I Might Say Tighten
You are going to want to frame the image fairly tight so that you do not get a lot of other objects in the frame that are peripheral to the main image. This means things like trees and buildings that are significantly closer to you. The reason for this is that they will either be too dark to see correctly or they will be picking up other light sources and will betray the light and color scheme that has been defined by the sunset.
Settings and Positioning
Zoom close enough to the sunset to frame it and the objects around it well enough. The brightness of the sun is going to change throughout the event so you have to be ready for this. If you have a digital video camera that will allow you to preset your shutter speed and iris numbers for quick change later you may want to do this according to what you observe from a previous night. You are going to want to set the camera in a clear location and then lower the iris below your normal position. The reason for this is that you will be able to darken the surrounding areas to maintain focus on the sunset, lower the brightness of the sun so that you can see the entire thing as a single object, and make sure that the image does not get blown out because your main object is a light source.
It is going to be especially important that you white balance ahead of time, but it is going to be difficult with the changing light. If you do not need to have consistent footage of the entire sunset in progress then it would be best to white balance periodically, but if you have to have it all you can do this at the beginning. You are not going to be able to get the light just right for the white balance both because the object you are focusing on is far away and is a color shifting light source. Instead you may just want to use a white card and do it according to the standard light of the area. This is going to make it somewhat lighter because your location is darker than the sunset, but that means you will just have to lower the iris a little bit.
Every sunset is different so you are going to have to position and adjust settings according to the one specifically. The most important thing to note is what you want in the frame, know that the sunset will darken everything around and brighten the image, and that you are going to have to zoom in farther and lower the iris.