Having done the planting I found myself wondering how I should have done it. I looked around online and found this video.
It looked like an interesting, albeit labour intensive, way of getting things planted, so I tried it with my leftover floating bits of the plant.
As I thought, it's tedious and labour intensive. I gave up after a dozen or so plantings. It was also a very effective way of getting the plants rooted - something I wish I had learned years ago. I tried it with a fragment of Ludwigia that had floated in the tank for the last two weeks (and grown a long root). It worked just as well. While I'm a little concerned that I might be burying the stems a bit too deep in the substrate (though, luckily, it's just gravel) I'm not too worried. I'll see how things work. So far though, it seems like a good technique for planting any small aquarium plants, especially those that come as cut stems with no real root mass. (I wouldn't, for example, try that technique with a sword.)