Let the flash feel direct, not destructive.
The familiar disposable-camera look comes from a small flash close to the lens: bright foreground faces, quick falloff, darker surroundings, and color that is not perfectly polite. Grain and compact contrast complete the impression.
Because a filter cannot invent physically correct flash, choose an image with frontal or nearby light. On a softly lit portrait, use a lower intensity so the recipe reads as compact-camera color rather than a pasted-on effect.