This kind of composite requires two steps: a) get the perspective and form correct, and b) use blend modes and/or opacity adjustments to make it look real.
Perspective and Form
Use the Free Transform tool for simple perspective adjustments.
Use the Vanishing Point filter for trickier situations, or where it has to look exactly right based on the perspective lines in the image.
Where the surface is curved, bumpy or is otherwise clearly not flat, create a Displacement Map from the best-contrast channel and use Filter > Distort > Displace to distort the image in a way that follows the contours of the image. Sometimes a combination of Free Transform and Displacement are necessary. See this question and its answers for more on this subject.
Blending the Composite
Sometimes, as in this case, switching to a blend mode such as Overlay or Multiply, or slightly reducing the opacity of the "add-on" will be all you need to do. It very much depends on the two images.
When you have it close to what you need, carefully check the lighting and texture. You may need to add a subtle highlight or shadow to make it real.