Mixing Flash with Ambient light its an esential skill every portrait photographer should have.
First, set your aperture, shutter speed and ISO to expose the background to your liking. Remember to use a shutter speed below the max sync speed for your camera. Avoid direct light on the face of your subject and if the ambient has some directionality turn your subject to face that way . This is the time when you work your composition and subject placement. The background got to look good and also the subject has to be placed in a good location for a balance composition
Once the background looks like you want it to, turn on your flash.
Then take a picture with the flash firing and aimed at your subject. I usually use a light meter but when I dont use it I start at 1/8 power or you can use any power you like or start in TTL for a base exposure.
Adjust the flash in small increments but aim to lower output and dont try to overpower the ambient
Fill-flash will help you control the contrast in your scenes and on your subjects. When doing so it’s important to maintain a good relationship between the ambient light and the flash. If you do so, the image could look quite natural .
Position your light to get proper light graduation on the face of your subject
If the face is properly lit and the rest of the body is too dark or the contrast is too strong than you need another flash to fill in
When balanced right your subject will have a nice glow but will not be contrasty and it will look very natural