The short answer is, either way will work. But… If the leaf has a curve to it, it’s best to place the leaf on the soil so that it creates a “hill” rather than a “bowl”. This will prevent water from sitting on top of the leaf as long when you water and can help prevent rot.
Placing the leaf this way will also put the leaf end closer to the soil, allowing roots to reach the soil more quickly once they sprout, without actually being buried in the soil.
Take a look at my weekly leaf propagation update to see how I have my leaves setup and how they are growing!