Graduated filters are expensive and we often forget to bring them with us. What to do if we are out already and need to use a graduated filter for a sunset scene?
The process is very simple: you only need to have something opaque (like a credit card, or even your hand!!) and place it where you want to expose less. For a sunset photo, you will put the card covering the sky, while the foreground will be exposed properly. Then, you will slowly raise the card when the sky exposure should start. After a couple of trials, you will get the perfect shot!
To calculate the exposure, take two photos: one where your foreground gets the proper exposure, and one where the sky gets the proper exposure. Then, you take a photo with the exposure for the foreground while covering the sky, and lift the card when the time left is the time you need for the sky. As simple as that!
Check the video for a practical example: