First, check that you have some. Approach the maturing pods on a dry, windless day and upend one onto your hand. The bottom of a stalk of Digitalis (foxglove) will be brown and packed with potential while the top remains unripe. With Umbellifers and Papaver, the whole head hits performance pitch at roughly the same time. Should you see only tiny spider skins and a couple of anaemic aphids, keep trying. If your palm contains a few nutty nuggets of future promise, proceed to step two. Go to the post office or corner shop and support a local business. Buy a packet of modestly sized paper envelopes, since shoplifting is indefensible and will not sustain the community. Return home, observing all speed restrictions and rules of the road.
Hypericum (St John's wort)
Hypericum (St John’s wort)

Back to step one, collect ripe stems upside down in paper bags which have been carefully labelled with each plant name and date of collection. Due to the proliferation of plastic in most retail emporia, you may find that such bags are as scarce as hen’s teeth. You may be obliged to make another trip, to acquire some large manila envelopes. This is not wasted time; if you store seeds in polythene, you retain any moisture present and this will make your booty moulder, rot and fail to germinate. Place the pods in a dry place such as an airing cupboard, to loosen the contents. Ensure that there are no holes in the container, or you will be picking poppy pips out of your underpants for the foreseeable future. When loose, shake the seeds free and remove any chaff, sticks, leaves or insect corpses which may fester and take your prizes with them. Put the good stuff in clearly marked small envelopes and store them in a box with a lid in a cool, dry place. I find that packets of silicone gel help to hold off water, thus improving shelf life. Swap your swag with other gardeners, to increase your stock for free.
Eryngium planum (sea holly)
Eryngium planum (sea holly)

To collect from soggy fruit such as tomatoes, the seeds should be rinsed in a kitchen sieve, so that no pulp adheres to them. They should be dried thoroughly while you ignore the comments of rude, vulgar family members who complain about your heaps of scrot littering the radiators and windowsills. You would be astonished at the ease with which exotic crops will sprout from a kernel. I have got lychees, paw paw and avocado to grow; it’s keeping the buggers alive that presents the difficulty.
Dahlia 'wittem'
Dahlia ‘Wittem’

Remember that the seedlings will be the result of cross fertilisation and may not breed true to the original plants. You could come up with something exciting and entirely original for little cost, other than pleasing the stockist of cheap stationery. Many home grown seeds will germinate best if sown as soon as they are shed from the plant. Unless you have the steady hand of a brain surgeon, you will spill a few in the course of detaching them. I make a point of distributing Hellebore, Aquilegia and Eryngium as soon as they are ready. It’s worth noting that the latter, sea holly, forms a sturdy tap root early in its development. If you sever this, the plant will sulk like a disappointed teenager, so if it grows in the wrong place, move it promptly. If you spot new leaves starting between paving slabs or under brick work, give the job to the adolescent in your life. It will give them a worthwhile task and keep them from bitching about the strange stuff you’ve got spread on drifts of newspaper, bunging up their access to lunch.