how to do
it is pretty straightforward. might be worth looking at some accounts of people's experiences with the early models (Brenda and Robert Vale in 'the new autonomous house' I think). They had some fruit fly issues and solved them with a fine mesh on the exhaust, and a 12v fan to ensure that ventilation always went the right way. that aside it's basically a box.
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McGill University (Montreal, Canada) published some DIY plans for a Clivus Multrum composting toilet (a clone: Clivus Multrum is a protected brand name) back in the 1980s. I'm sure that you can run a Google search and find equivalent plans today.
Best of luck.