To use your Feed Mill, click on it. A menu will pop up, showing you the harvested crops you currently have available to use.

Individual crops yield different amounts of Feed when placed in the Mill. This amount is displayed in small text under the crop’s image.

Clicking on a crop will add it to your Feed Mill. The total amount of Feed you are going to receive shows up at the bottom of the Feed Mill menu. Every time you click on a crop, it will add to this number.
Clicking on “make” will turn the crops you selected into Feed for your animals. If you accidentally add a crop to the Feed Mill on mistake, click on the small “X” button at the top of the Feed Mill menu. This will let you start the process over, without using the crop you accidentally selected.
Super Feed
Aside from the regular Feed that you can make from the Feed Mill, there's also a new Super Feed Feature in the game!
We've given everyone a brand new Feed Mill that produces Super Feed! Super Feed is a new resource that allows you to Feed your animals faster and earn triple resources.

To make Speed Feed, click on your new and improved Feed Mill (now known as a Super Feed Mill) and click on the "Super Feed" tab.
From this menu, you can see that a certain amount of regular Feed is required to make Super Feed. Click on the "Feed" tab at the top to turn your crops into regular Feed. Once you have enough regular Feed, select the number of Super Feed packs that you want to buy and click on the "Make" button. After you have made some Speed Feed, you have to wait a bit until you can make more. The timer in your Super Feed menu shows how long you have left. If you don't feel like waiting, you can always purchase Super Feed from the General Store.
Super Feed allows you to feed your animals faster by skipping the timer between feeds. Use Speed Feed to Prize you animals quickly for the most points at the County Fair!