Plumeria seeds should be planted at a shallow depth for germination. Here are the general guidelines for planting Plumeria seeds:

1. Depth: Plant Plumeria seeds at a depth of approximately 1/4 inch (about 0.6 centimeters) into the growing medium. The key is to ensure that the seeds are covered with a thin layer of soil. You can also insert the seed into the soil, leaving the wing out of the soil.
2. Growing Medium: Use a well-draining potting mix that is suitable for Plumeria seeds. The mix should be loose and friable to allow for easy root penetration.
3. Container: If you are starting the seeds in containers or seed trays, fill the container with the potting mix, and then make small indentations in the soil to the recommended depth for each seed.
4. Spacing: Space the Plumeria seeds evenly in the container, ensuring that they are not too close together. Providing enough space allows each seedling to develop without overcrowding.
5. Covering: After placing the seeds in the indentations, cover them with a thin layer of the potting mix. Gently pat down the soil to ensure good seed-to-soil contact.
6. Watering: Water the soil lightly after planting to settle it around the seeds. Avoid saturating the soil, as Plumeria seeds are sensitive to waterlogged conditions.
7. Maintain Moisture: Keep the soil consistently moist but not overly wet during the germination process. Using a misting spray bottle can help maintain moisture levels without disturbing the seeds.

Plumeria seeds are relatively small, and planting them at a shallow depth allows them to access sufficient oxygen while still being in contact with the moist soil. This depth also allows the emerging seedlings to break through the soil surface easily as they grow. Remember to provide the appropriate temperature and light conditions as well, as these factors are crucial for successful germination.