All you have to do is cover them with soil....just a light covering as deep as the seeds are in size. When I grow Catharanthus, I dust a layer of my germination medium on top of the sown seeds and water them in with a misting nozzle.

The problem is that Catharanthus takes a long time to germinate and develop unless the soil is warm, at least is my experience. My heating cables have an automatic thermostat that keeps the germination flats at about 68F.

I used to know how long it takes for outdoor soil to warm up once the air temperature began to rise but I've forgotten the calculation.

Get yourself a row cover. They are about 4 or 5 feet by 25 feet. They are not very expensive and they will warm the soil and let the light in and water in. Remove a bit of soil. Water the ground and plant your seed. Then give them a very light covering of reserved soil. A few upended pots or hoops will keep the row cover from flattening the plants as they emerge. Cover them with the row cover and secure the sides and ends with soil or stones or wood so the light row cover doesn't blow away. Be sure to water as the soil tends to dry with the heat created by the sun through the row cover. Row covers raise the air temperature under it and the soil temperature. They retain 80% of the light and moisture goes right through them but it does make the light fabric heavy and that's why I suggest upended pots or small hoops. You can make hoops with wire clothes hangers. With wire cutters clip off the hanger part and untwist the top. Straighten the hanger and bend into a hoop.

I tried an experiment with row covers one year. I had 2 perennials that were the same variety and roughly the same size. I cut a piece of row cover and covered 1 as they were emerging from the soil. In 3 weeks the 1 I covered was more than twice as big as the 1 left uncovered. I removed the cover and that slowed down it's growth and eventually they were both the same size but it did prove to me the worth of row covers

Thank you! I’m going to order the row cover now. I never knew they existed. Thanks so much!

Great advice about the row covers! I use them all the time for my veggies, most of which are planted in situ but didn't think to recommend them for the OP's question.