The idea is simple:
Start with a copy of Bungston's home solar charcoal distiller [see link] (summary: made using a big fresnel lens, a kettle style charcoal grill, a special top to the kettle with metal hose attached, and a bucket of water to cool the distillate.)
Load the kettle up with raked
up leaves, lawn clippings, and other yard waste, set it beneath the lens, and cook until it stops giving off volatiles.
Let it cool, and you now have a pile of biochar -- it will be crumbly and dusty, and probably unsuitable for cooking over directly, but there are lots of other uses for it.
You could make charcoal bricquettes with it, as in django's idea [see link].
You could mix it into your compost pile, and make terra preta.
You could scatter it on your garden, and rake it into the top of the dirt.
You could even just through it out (since it's reduced in weight and volume, you save money on garbage bags).