I have two basic approaches -- neither one of which works well for sunflower seeds, I haven't got that one figured out, though it would be easier if I was only growing one seed-size.
1) Like Matt mentioned, I winnow by pouring from one bowl to another on a windy day. Some seeds get away, but mostly, the chaff blows off. I haven't ever gotten out a fan to simulate wind, but I've been tempted. It takes a little practice to figure out how to aim the main stream of seeds so that it makes it in the bowl, especially as the wind shifts direction or strength, but it's not too bad. When I'm doing this, I combine in Eino's swirling the bowl suggestions too. If you can get all the chaff to percolate to the top, you can scoop or blow it off and then the winnowing goes more quickly.
2) I also use a set of graduated mesh sieves like this:
https://www.amazon.com/dp/B0946BX45M. Depending on your seed size and the characteristics of the chaff you're dealing with, you can use it to let the seeds fall through and leave the chaff behind, or vice versa. Or, honestly, both in different orders.