Shooting against the sun
You need, Any Camera, Any Lens, and very important, one light.
Most of us invest in more lenses and we forgot about the light .
Why is so important to have light in your camera bag?
You never know what light condition you will get. You go to location with your model and there's too much shadow, too dark, or sky full of clouds and everything is flat. Or too much sun.
We have option to use reflector but reflector we cant control like speed light, And usually reflector requires another person to hold it.
So, Light is the Solution!
Simplest way to shoot model in the sunset is. Set your camera like you shooting landscape. Ignore the model and don't use the speed light. Make sure your sky and ground behind the model is correctly exposed. you can drop your exposure to achieve more dramatic look or just keep it like it is. Make sure you don't burn your sky . Take a shot. you will have background like you want it but Model will be in the big shadow, because sun is behind her.
So what next ? point the light at the model, set that light to 1/2 power or full power. what ever you want. and shoot. if model is still dark set more power of your flash, try few times and adjust the light to satisfying result .