Keep it simple. The first thing you want to control when shooting models against sunsets is your natural light exposure. The setting should be exactly how you want the background to be. This will begin to change as the sunset moves into dusk.
Let me explain further from a one shot scenario. Set your camera to manual mode. Meter the scene as if you were taking a picture of the sunset the way you want that to look. Pay attention to your exposure when you preview your shot. Generally speaking, if you are not shooting high speed sync, set your camera at your highest sync shutter speed when making this exposure. Let's say, for example, your shutter speed is 200 and your F-stop is f-11 at iso 100. After you are satisfied with your exposure setting to get the look and feel you want of your background; you will then need to set your flash output to allow the flash exposure to be at F-ll. That is so simple and yet many photographers are intimidated by this. The same technique is used with high speed sync capable flashes and cameras. The only thing you need to think about is the power of your flash as it takes more power with HSS. Let's repeat. Step 1. Find your exposure that you want your background to look like. Step 2. Bring in the flash. Adjust the flash power to meet that exposure on your model. It's as simple as that.
As your natural light changes with the setting sun, you need to know that the ambient light exposure of your scene will change as the sun goes down. You control your ambient light with your shutter speed since the flash exposure is controlled by the F-stop (aperture).
Let's say you are not working with HSS. Keeping the exposure at F-11 and the flash power the same, you can adjust your ambient light exposure by the shutter speed. This will not affect your flash exposure. From 200 shutter speed you can go down in increments to a safe hand holding speed. Since flash can freeze certain actions, you can generally get away with shooting as low as 60th of a second shutter speed. Again, your ambient light is controlled by your shutter speed and your flash exposure can be controlled by your aperture or the power of your flash.
You have to watch out for ghosting of your subject with slower shutter speeds. Especially if your subject is moving. One of the advantages of working in High Speed Sync is you have lots of room to work with in safe shutter speeds. Generally, the light has to be closer to your subject when working with HSS.
Once you get to that slow shutter speed threshold, you will need to start thinking about opening up your aperture then to get your sky the way you want it. When you start opening up your aperture, you will need to turn down the power of your flash to balance your subjects exposure. This will allow you to control your flash exposure on your subject and keep you from any ghosting of your subject against the sky.
At anytime. You can adjust accordingly with the same starting method of setting your shutter speed to work with your flash in sync or HSS, metering your natural light scene, adjusting the F-stop (aperture) to meet your desired ambient light exposure, and then adjust the power of your light to match your natural light scene exposure.
Pray for some clouds. A clear sky is not your best scenario to photograph sunsets. You want some clouds for an epic sunset. Start paying attention to the late afternoon to see if the conditions are going to be right for a wonderful sunset. A blanket overcast sky will kill it altogether and be a bust. If the sun setting is not going to be ideal, take that time to simply shoot sweet light or get creative with sun flares, rim light with sun and silhouettes. Always make the most of any situation.
Power Shoot. Shooting at sunset is what I call a power shoot. You only get about 30 minutes from sunset to early dusk for a usable background. This is a thrill and no time to be fumbling around. You must get the shot. Especially if it is an epic sunset with a wonderful model.
Don't give up when you think it's not going to happen. I have seen the sun break through an evening overcast horizon on moments notice. Get set up and pay attention of the western sky to see if it has the possibility to change. The cloud blanket might break up just in time and you don't want to miss that opportunity of the sun filtering through. That moment could be an amazing scene that lasts for only a few seconds or if you are in luck, a minute. It could be that second you will nail the image of your lifetime.
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