This is what we found when our seeds grew quite large inside the bag, only to rot later. Sometimes there are many ways of growing the same thing. Fresh seeds generally do not need special treatment but as the 3 month period after collection has passed they may need a bit of help as some have outer skins that harden. In general cover seeds the thickness of the seed, a tiny seed needs barely covering. Press them firmly into seed mix.
Seeds prefer a little shade while starting out.
Gradually harden seeds off to the air by uncovering slowly. Do this over several days before pricking out or prick out and move pots out slowly. Most seeds do not like to be kept in overly hot conditions. Nicotianas do like it hot. Use a sterilised seed raising mix.