You should put it about twice as deep as the seed is large. For example, if the seed is 4 millimetres long, plant it 8 millimetres deep. Try planting it "pointy end " down.
One end of the seed has a flattish profile from where it was attached to the plant. This is the end that you should be able to see once you have planted it in the soil. I use a small pointed stick, something like a bamboo skewer works well, to first make a small hole in the soil. Drop your seed next to the hole and using your pointed stick, get the seed to go down the hole pointy end first. Then use the stick to push it in to the desired depth. Cover the hole with soil and pat the soil down gently with your fingers. You want to make sure the seed is in good contact with the soil, so pat the soil down gently but firmly too, if that makes sense. Just not so firm as to push the seed in any deeper. If you do not "pat" the soil down firm enough, your seed may float to the surface when you water it. Try using something with a "rose" on it so the water comes out like in your shower, in sprinkles. Be quite generous when you water it the first time, after that try to keep the soil moist. but not wet. Do not put anything over the seed/pot, like plastic film/plastic bottle/tupperware container; this will cause too high humidity and your seed may rot or get fungal problems. Your seed also need to be able to breathe, believe it or not, and covering your seed/pot with something will suffocate your seed; your seedling too once it sprout. Cannabis plants did not evolve in sweat lodges, so do not cover them with anything. Keep your newly planted seed in its pot in a warm area, with gentle overhead light, but try and keep it out of any winds or drafts as his will dry the soil out faster. After anywhere from 2-10 days (so don't give up too early!), your happy little plant should be up through the soil and looking for life giving light. Once your seedling has opened its first baby leaves, it needs to go under your lights or out in the sun. They can take full sun from day one as this is how nature works, but if you are using electric lights, make sure they are not too close otherwise they could burn your newly hatched baby. This distance all depends on what sort of lights you are using and the strength of the lights. Generally with LED lights it is around 24 inches or so, with fluoro lights about 12-14 inches and with HID lights, at least three feet. Hope this helps.... Organoman.