The first step is to gather up the components for your design. Below is the wood background I used. If you need yours to be a higher res image or just would like to try a different background, check out one of the many great stock image sites out there. One of my favorites is stock.xchng, and it's free! That's also where I found the fruit photo I used. You can download that at this link.
Change the blend mode of the highlight layer to 'Lighten'. That will make only the white part of the layer visible. Now you should have some nice highlights displayed. Also, add a layer mask and using a soft brush at moderate opacity brush out some of the unnecessary highlights. Especially around the top where the zipper will be. You can also play with the opacity of the layer itself to temper the highlights. You can really overdue the whole look of the bag by leaving too many highlights. So be careful to brush out the right amount, and remember because you are using a mask, it is quite easy to switch your brush to white and paint some back in if you take too much out. Here's what I got at this point.