Easiest way to do it is one of the following:-
1) For seeds that are a similar size to sweet peas or have a seed head with tiny seed at the bottom - go to a chemist and get a pair of small blunt-ended manicure tweezers so you can pick each of them up without damaging or dropping them and just pop them in your pot.
2) For seeds that are smaller (like onions or leeks) - get your pot filled with compost, open one end of the packet, hold it with one hand, then tap the seeds out into the crease of your palm on your other hand. Hold this hand at a downward angle, tap the base of your thumb, and the seeds will go down the crease and fall onto the pot. Move your palm over the pot as you do this so they are fairly evenly dispersed. These types of seeds can be individually pricked-out when they get going.
3) For tiny seeds like carrots or any others that look like grains of sand - get a polybag, put a small bit of sand in it, add your seeds, seal it with Sellotape, give it a good shake, open the bag then pinch some out with the tips of your fingers and sprinkle it over your pot. Job's a good-un and won't cost you a quarter as much as a gadget would.