I only have two Fennel plants, but they are quite big, and Fennel produces a lot of seeds. This one is top-heavy with the weight of its seeds:-
Harvesting was a snip. No, literally: "a snip"! Two minutes' worth of work with a pair of secateurs, cutting off and collecting the seeds-heads. Here you see them arranged in a bowl simply for photography purposes. Since then they have been put into a paper bag and placed in the airing-cupboard to dry. [Don't use a plastic bag for this - it will sweat and the seeds will go mouldy.]
It will probably take about 10 - 14 days for the seeds to be completely dry. Then I will strip them off the flower-heads and store them in an airtight jar.
Fennel seeds are used a lot in Italian cuisine, especially as an ingredient in salami, but it is also used in many recipes for the great British Black Pudding. In Indian cuisine they are often coated in garishly-coloured sugar and offered as a "mukhwa" or digestif to eat whilst drinking tea or coffee after a big meal.