Growing up I was always told the secret to seed raising success was "warm bottoms, cool tops!" The ground temperature needs to raise above 12 degrees for the right conditions needed to germinate. I have great success with my heated propagator for the germination but there are certain steps I always follow.
1. I fill the tray I'm using to the brim with a good quality seed rising mix and dampen down with a fine mist spray
2. Firm this down using a flat tool then sow the seeds carefully between the finger and thumb. That way you can spread them more evenly than sprinkling them straight from the packet.
3. Cover the seeds no deeper than the thickness of the seed. In fact some of the smallest seeds I don't even cover.
4. Water again with a spray mist but don't soak as this can cause the mix on top to form a mat that the seeds can't lift their heads through
5. Cover the seeds with a clear lid. This helps keep the humidity around the seeds
6. Position your seeds where there is plenty of light but not direct sunlight as this is too harsh for the young seedling
7. When the seedlings appear open the vents on the lid if you can to reduce the humidity. Eventually you can remove the lid altogether.
8. When the seedlings are 2-3cm high "prick" them (transplant) the little seedlings into little pots to ensure they continue to thrive. If you don't do this they have to compete with each other for food, oxygen, water and sunlight and you end up with spindly weak seedlings
9. Remember to "harden" them by exposing them briefly to the outdoors each day before putting them outside.
10. Watch the weather before you plant them out. Young plants are particularly susceptible to cold and frosts so give them the best start by protecting them from the cold