How do you use Scarcity with your own product?

Call me pedantic, but copywriters and marketers actually refer to that as "urgency", rather than "scarcity".

It's rather a different point. There can sometimes be overlap between the two concepts, it's true, but "scarcity" - as discussed in this thread - intrinsically relates to limiting the supply, not to adducing reasons to buy something quickly.

This is exactly the point.

The people selling digital products on Clickbank claiming permanently on their sales pages that "only 9 copies remain" (that's an example of scarcity, not urgency) are typically losing large numbers of sales with this "tactic" (to use a polite word), are genuinely not perceptive enough to realise it and have never tested it themselves (or they'd know better). Typically, it never occurs to them that "looking so obviously like a liar when you're trying to sell something" is a bad thing to do. Some of them may even not realise that it's obvious to the average prospective customer that they're lying? Who knows what - if anything - goes through these people's minds?

It's not about what you call "it". There isn't an "it": those are two different things.

Most marketers who produce "home-made copy" (i.e. not written by a professional copywriter but written by themselves after reading an introductory copywriting book or two, and/or maybe getting some input from "writing friends" who are also not professional copywriters) tend to shoot themselves in the foot with it, and not to appreciate that both of these techniques, when unwisely used, are actually directly and strongly counter-productive. People imagine that "they must work (:rolleyes, otherwise other people wouldn't be using them all the time".

The reality is that there are correct/good/beneficial and dishonest/contrived/counter-productive ways of using each.

The one you've asked about, "scarcity", is in fact by far the more widely mis-used one of the two. (Out of the times that it's used at all, I mean). It's actually pretty difficult, with a digital product, to justify plausibly. So it's perhaps not too surprising that so many people who try it manage to make a complete mess of it and end up losing business by having tried.