You can purchase these mills from the Appliances > Kitchen Appliances section of the catalog. They can only be placed on a counter.
Clicking on the mill will allow you to select from several different crafting menus. Once you've chosen what you want to make the mill will start grinding for about 30 sim minutes until it's done.
The electric mill has a little bag on it that slowly fills up until it's done but you can always hover over the appliance and it will tell you it's status. I even made a custom sound to for these so you can hear if the mill is still working! This mill requires power to function.
The grinding mill works the same as the electric mill except it doesn't have a bag that fills up while it's grinding. This mill is will work with or without power so it's a must for those sims that like to live Off-the-Grid!
Once the mill has finished you can click on it and choose to package your ingredients. Your sim grab their newly crafted ingredient from the mill and put it into their inventory.
Please keep in mind you must have the custom ingredients installed for these crafting menus to show up.