If it's an F1 hybrid type, and most sweetcorn are nowadays, then you will probably not get the same result. Just leave the cob on until it is fully ripe and before the earwigs and birds start to get at them. Store the whole cob somewhere dry and then scrape the kernels off during a cold winter evening when there's nothing on the tele.
I'm growing corn from seed taken from an F1, this year, and the plants look absolutely fine and are growing well. I'm waiting to see what they will taste like. My guess is that if they are picked at the right stage they will still be very sweet, but they probably won't stand for longer like some of the true F1 types. F1 sweetcorn seed is so expensive now for so few, that it's well worth trying. Make sure your corn is growing in isolation from other types and not likely to be crossed with maize as the pollen can travel quite a way in the wind.
Parsley can just be left to flower and seed by itself, and it often self-seeds easily if you keep an area around the plant tilled and weed free. Rocket makes masses of seed and is easily collected when the little pods turn brown, or you can cut the whole plant and hang it to dry indoors. Put a sheet of newspaper underneath the hanging plant and the seed will fall out of the pods when ripe.