Not too hard,
You using photoshop?
first have the three images you want to work with in a document, the reflection layers, and the glasses layer.
Set up the layers where you want them, so reflection 1 over first lens, and reflection 2 over lens 2,
::hide both reflection layers, (eyes next to the layer name) make a selection of the glasses lens 1 or 2 and then unhide the reflection layer, select the reflection layer you are working with in the layers palette, make sure its active, now in the document, hit ctl ,shift, i (or select / inverse) which will invert the selection, hit delete , now you should be left with a image the shape of your glasses lens 1 or 2 depending on which one you worked with. DO the same for the other lens, (select, active, invert selection , delete)
::: Now to make the image more round looking you would have to play with some filter, give this a go, select the relection layer, go to the filters/ distort/ pinch (or sphereize) , play around with the setting to get a nice boldge feel.
A nice trick to get some reflection on top of this image is to select the lens selection you have, make a layer on top of the reflection image, select the gradient tool, make the gradient white to transparent, select the linear gradient , and from the top left to the bottom right of the selection make the gradient, its kinda like making a highlight to a object.