I'm assuming it's an older iguana? It's really hard to see the quick the older they are. Unless anyone has some other ideas, you'll have to do it just as you would any other animal with dark nails: a little bit at a time. I'm not sure how his temperament is, but in any case, I'd restrain him in a towel and get him in as bright a light as you can manage. If they're really gnarly, you can usually tell where the live part of the nail is. There should be a little ridge if you look from below the nail. Trim to just in front of that point. Also, keep a styptic pen or powder on hand in case you go too deep. If he's still a young iggy, you may need to trim every 3 weeks or so, or whenever you notice they're growing.