The cutout is 20% of the job

Most failed composites have perfectly good masks. What gives them away is that the two images were made under different conditions, and nothing about the mask fixes that. Work through five things in order.

1. Perspective and camera height

Match the horizon line first. If the background was shot from waist height and the subject from eye level, no amount of colour matching will help — the viewer reads the mismatch instantly even if they cannot name it. Choose source images with compatible viewpoints; it is far easier than correcting one later.

2. Direction and quality of light

Light must come from the same side, at the same hardness. Hard sunlight gives sharp-edged shadows; overcast gives soft ones. You can shift direction a little with dodge and burn, but flipping a subject lit from the left into a scene lit from the right rarely survives scrutiny.

3. Colour and contact shadows

Clip a Curves or Colour Balance layer to the subject and pull it towards the background's colour temperature. Then add the contact shadow — the dark line where the subject meets the ground. Its absence is the single most common reason a composited object appears to float. Paint it on a Multiply layer, soft brush, low flow, tighter and darker at the point of contact.

4. Focus and depth of field

A tack-sharp subject on a background with shallow depth of field looks pasted. Convert the subject to a Smart Object and add a small Gaussian or Lens Blur to match the background's focus falloff at that distance.

5. Grain and colour depth

Every camera leaves noise, and two images will not match. Add a grain layer over the whole composite at the end — a 50% grey layer set to Overlay with Filter → Noise → Add Noise, monochromatic. Unifying the grain does more for believability than almost anything else at the finishing stage.

Judge it small

Zoom out to fit-to-screen, or shrink the image to thumbnail size. Compositing errors that hide at 100% jump out at small sizes, because you stop reading detail and start reading the overall shape and tone.