First take an image of what you want to chrome.
Next you need a chrome ball, I usually use flickr advanced search 'chrome ball'.I found a pretty decent picture of a Christmas bauble.
Then I masked it out and softly dodged the grey reflections.
So to blend the chrome ball into the apple's environment.
Now for the chroming.We'll start with the smaller details, like the bump on the top left of the apple.
Use a feathered selection as shown to copy paste the edge of the chrome ball that fits the edge of the bump and distort it onto the bump using distort/warp (phoenix users remember to zoom to 100% before using distort- to avoid artefacts).Note--The dark glow from the chrome ball is only to emphasis the edges, do not apply a glow to your project.
Repeat on the smaller bump next to the apple with the same shape.
For surfaces try to take into account where the surface would be if it was on a sphere.Here I got lucky and the surface and the edge were on the same layer.
Here I had no such luck.
Note that the bottom selection should overlap the top one on the apple.
In this one you need to distort/warp the selection to bent it in the opposite direction.
Like this:
Instead of copying the surface and the edge on two separate layers, you can select them both. Then, on the new layer, select the part that overlaps, and distort/warp it onto the other part, as shown.
Keep putting selections of the chrome ball on the edges of the apple, take into account bumps and flat surfaces, like the one on the apples right.
Now to finish off the chrome.
Distort this selection onto the apple.
And here is the finished chrome:
Reflections, shadows and highlights can all be found in different tuts.
Not much, I know, but with more complex objects like a face, it works wonders.