What sculpting mediums do you have to work with? The material can determine the best way to approach the job, for example if you're using Kneadatite (GS) you might want to avoid having to pare down or sand, but you could rely on carving, filing and sanding for a lot of the work with something harder like Milliput or MagicSculp.
The basic method I would use is roughly the same with either material though, just the details of timing and how you work are different - press more putty than you need into place and start forming the shape using your sculpting tools of choice, paring down and refining as you go.
In theory you could form goggles in one pass, but you might want to do the lenses separately to keep the flat bits as flat as possible, sculpting all or most of the surrounding elements afterwards (in more than one step if necessary). It's well worth considering doing the lenses in plastic card or anything else hard and flat rather than trying to form them in place on the model.
Einion