Now!

I sow all self-collected seed immediately. If you have a lot, Snoop, you could hold some back for a second sowing in spring.
Otherwise, as with Owdboggy, Now.

Agree with Owdb and Bulba - think about what happens in nature and try to emulate it if possible.

I have grown dozens of varieties of Alliums from seed over the last cough cough years. It is reasonably easy, but the bigger the bulb the longer the wait for flowers. So here goes
Sow the seed fresh. It germinates better straight from the plant rather than being dried off.
Cover lightly with grit, gravel or whatever rather than compost.
Sow in a deep pot rather than a seed tray (reasons later).
Leave exposed to weather. They need a period of cold followed by warmth to initiate germination.
When (if?) they germinate do not be in a hurry to p rick them out. This is the reason for deep pot rather than seed tray. They have only one root to begin with and if it is damage, it dies and a new one has to be produced from the base of the seedling. usually they die.
Feed the seed pot with dilute Baby Bio type stuff, until the leaves go yellow then allow the pot to dry off.
Repotting may be done when they are dormant. Some types never really go dormant so be careful.
I often do not repot until they have had another seasons growth. Remember many of them actually grow in late winter/early spring.
Cannot think of anything else for the moment.
Not as hard as it seems. Oh and since they are almost always self pollenated you get what you sow, no variations. UNLESS you have been given some Allium flavum hybrids in which case.................

Hi Steve, personally I wouldn't bother!!!, life is too short, unless you want to grow a rare species and increase your stock, I would just buy the bulbs and plant them, and.get flowers next year, Derek.

I am fully with Snoopdog on this, Derek. I would suggest that a high percentage of the plants in our garden have been grown from seed by ourselves. The advantages:
Seeds are cheap
Instead of buying one plant for, say, £2 you could get a nice clump of a dozen from the seed packet.
Excess plants can be swapped, sold or donated to local events
By being a member of a specialist society (Scottish Rock Garden Club)k of some 5,000 varieties of plants, most of which just aren't available in the garden centre.
It is fun
"it,s just something i do now and then just out of interest" ( well, maybe a bit more than 'now and then' in our case!)