It's simple. There's an alternate tab you can see on the screen when cooking. It's on the side of mobile devices and above on PC and PS consoles. Just move to that tab and select which ingredients you want to process. You can only make 99 of the selected ingredients at a time. You can make 99 flours while also processing meat and salt into 99 ham, so the queue only works per resource. Even if the whole batch isn't finished, you can collect the processed food and add to the queue (if it isn't full yet). Each food has a different processing time, so you'll have to wait longer for the more complicated ones.