You can improve a pop-up flash a little, but there are limits. Because the flash is very small and close to the lens, it will never look as soft as a larger off-camera or external flash.

Practical options:

• Use a white card at about a 45° angle in front of the pop-up flash to bounce light toward the ceiling. Some people cut small slots so the card mounts on the flash itself.
• Use white paper or similar DIY material in front of the flash to diffuse or redirect the light.
• Buy a dedicated pop-up flash diffuser such as the Gary Fong Puffer or Lumiquest Soft Screen.

Important caveat: these solutions are generally “better than nothing” rather than dramatic fixes. Diffusing such a small flash reduces output, and the light source is still small, so the improvement is modest. Bouncing off a nearby white ceiling can help more than simple front diffusion, if the room allows it.