The four failure modes
Almost every bad face swap traces back to one of four source problems: the face is angled away from camera, the lighting is mixed, something occludes the features, or the photo is too soft for the detector to lock landmarks.
Fix the source and the output fixes itself. That is the entire secret, and it is worth internalizing before you blame the tool.
Angle: square to the camera
Front-facing beats profile every time. The landmark mesh needs both eyes, the nose bridge and the full jawline. A three-quarter angle is workable; anything past that forces the model to invent half the face, and invention is where glitches are born.
Tilt counts as angle. A portrait shot with the chin lifted high will swap onto a level template badly, even if the face is pointed at the camera.
Light: one direction, no drama
Studio-style, single-direction lighting is ideal. Hard side-lighting that leaves half the face in shadow, colored nightclub lighting and strong backlighting all corrupt the identity signature the encoder builds.
If you must shoot indoors, face a window. It is the cheapest softbox you own.
Occlusion and sharpness
Sunglasses, masks, hands on the chin, hats cutting the forehead: anything covering the features removes data the model cannot recover. Hair falling across the face is the most common hidden offender.
For sharpness, tap-to-focus before shooting and check the crop at full zoom. If the eyes are not crisp at 100%, the swap will soften them further.
The pre-flight checklist
Front-facing, both eyes visible. One light source, face toward it. Nothing covering eye-to-chin. Eyes in focus. Shoulders in frame for context. Five seconds of checking against this list eliminates the vast majority of glitchy results.