dont let gravity do its work, i've done this and it causes the stem to have a weird bend at the bottom. Use a pencil to make an insert the length of the root, then gently put the seed into the hole and cover with a thin layer of soil and spray water on top. I wouldn't use tweezers as its difficult to know how much pressure your applying on the seed, just use your hands, your finger tips are softer than tweezer ends and be gentle. This is what i do and never had a seedling not poke up through the soil, good luck