Seeds come in many different sizes and shapes, each needing different conditions.
For tiny seeds, use a good quality, fine seed raising mix and scatter on-top. Don’t cover them as they will get lost under all that soil. Lightly mist with a fine spray of water to prevent them from being washed away.
Small to medium seeds can be covered with soil and watered in.
Large seeds need to be buried twice as deep as they are wide to allow plenty of space to shoot upwards.
Some seeds need to be soaked in warm water, chilled (stratified) first or scarified (scratching or damaging the outer coating) to aid germination. Check what requirements the seed has before sowing to maximize chances of success!

To be a good seed-raising mix you are looking for a lightweight, sterile, fine, well drained mix.

It also needs to be able to hold moisture in order for small, delicate roots to develop easily.
Materials like perlite, sand and coco coir are fine and add drainage. Light seed raising mix prevent seeds from becoming compacted and allows roots to breathe. Good drainage stops seeds and roots from rotting.

Ability to retain moisture allows seeds to stay hydrated. Slow release fertilizer can be mixed through to give a boost of nutrients to growing seedlings. Adequate watering is also key. Moisten mix before sowing. Give trays a tap, moisten then sow to remove any air gaps. Bottom sowing can also help moisten the mix.

Most seeds need lots of light and heat to germinate. This could be in the form of a tunnel house, heat pad or mini green house or even a sunny windowsill with lots of light.

Lightly water baby seedlings with mist or lightly shower to ensure they’re moist but not drowned!
Light is important - bright intense light unless otherwise stipulated. Bottom heat can also be added to aid germination.

Once seedlings have popped up, they need good air flow and feeding them will help them get going. Be careful when feeding seedlings with a liquid feed, less is more. Seedlings can also be hardened off before planting out to gradually introduce them to outside temperatures.