When you set up a sell order in the normal way, a check box will appear. Checking it will turn the sell offer into a automatic offer instead of a one-time thing.
Once you have an automated offer set up, they will appear at the bottom of the market page. Next to the order is a Cancel button that swill cancel the order.
There is no way to edit an existing offer. Just cancel it and make a new one.
If you want to experiment, just set a huge threshold, like 5000 bushels, and make one. It won't trigger, but it will let you see what they look like.