It sounds tricky, and it is a bit because guesswork is involved, but as you shoot more and more pictures, you will find that your guesswork becomes more refined, and you can usually estimate an acceptable exposure. If your subject is backlit or sidelit, you may also have to make an exposure compensation. Keep in mind your subject's tonal brightness. A white or light-colored subject will need less exposure than a dark one. There is another way of estimating exposure when you don't have a light meter .
Film is packaged with the manufacturer's exposure guidelines for different lighting circumstances.
These are printed as a film datasheet either on a slip of paper inside the package, or actually printed on the inside surface of the film carton itself. If you compare these recommended exposure settings with the Sunny 16 rule, you will see how close they are to each other. The Sunny 16 Rule and the film manufacturer's data sheet both provide ways of checking the accuracy of your light meter. More importantly, Sunny 16 gives you a reliable rule of thumb for calculating exposure when you don't have a light meter or when your light meter stops working.