A good starting point is to shoot in full manual so you can control both ambient and flash light separately.

Try this approach:

• Set the flash to a fixed manual power, such as 1/4 or 1/8.
• Use your camera’s maximum flash sync speed (around 1/200–1/250) to reduce ambient light.
• Adjust the aperture to control how bright the flash-lit subject appears.
• If needed, fine-tune by lowering or raising flash power.

For learning, change only one setting at a time so you can clearly see its effect.

Another valid approach is to expose normally and then darken the image in post-processing, which often gives you more precise control over where shadows fall and what parts of the subject stay highlighted.

If your shots are still too bright, the likely fixes are: reduce flash power, stop down the aperture, and keep shutter speed at sync speed to suppress room light.