Start by filling your punnet almost all the way to the top. Tap the punnet to level the seed raising mix. Press a finger lightly into each cell to make a hollow for seeds to sit in. In larger punnets, you can make grooves -- otherwise, the seeds tend to roll to the edges of the punnet.
I'm right-handed, and I sow seeds like this: pour seeds into the palm of your left hand. Pinch a small number of them between your right finger and thumb. Roll your fingers over a punnet cells, enabling 3-6 seeds to drop into each cell. Why so many seeds? Some won't be viable, and some won't produce strong plants. Later on, you'll pick out the seedlings you don't want, and just keep the strongest ones. Unlike seedlings, seeds are cheap, and you get lots in a packet! How many broccoli seeds can you see in this punnet cell?
Mistake 1: Burial. One of the commonest mistakes is to cover your seeds too deeply. The little seeds we put into punnets are generally the kind that land on the ground in vast numbers near the parent plant. They don't need, nor want, a heavy blanket on their heads. Sprinkle some seed raising mix to cover the seeds up, but no more than you need.
I planted up a punnet of snapdragons yesterday. The seed was like dust, and the packet said to just sprinkle the seed on the surface. Even seed raising mix can be too heavy for such tiny seeds.
Mistake 2: Drowning. I stand my punnets in trays, and water from below. When the surface of the punnet is dry, I add water to the tray. It is OK for the punnets to stand in water for a few days at the start. When seedlings emerge, however, they are susceptible to mould ("damping off"), so make sure you only water seedlings when the seed raising mix is starting to dry out.
Now for some recent mistakes of mine:
Mistake 3: Invoking Phil. These beetroot seedlings only get morning light, and that's not a big problem -- the sun is terribly strong here at this time of year. The problem is that I haven't turned the punnet around to keep the seedlings upright, so they're all lopsided. (After I took the photo, I turned them around, and they look fine today.)
Mistake 4. Complete Forgetfulness. I really truly thought I'd taken all the planted-up punnets out of the shed last week and put them on my porch...
These are pak choy. The residual moisture in my rather dry seed raising mix was enough to get them going. Good germination rate, you may notice. The seedlings have reeeeeally long stems because the only light source in my shed is the east-facing window... across from the bench!
The previous post in this series is Ingredients and the next one is After Germination.