Here is how it works:
Right corner,the fork and knife icon saying "auto-eating"
click on add set,choose whatever you want to eat,then click on the arrow-in-the-middle-top-which-goes-right and ta dah! You can now click on the X on the right corner of that little window and close it,whenever you feel like eating something manually and fast,just click on auto-eating and then click on the food. You don't have food which restores your health and mana? Don't worry,click on add set again,then choose the food at the right column,choose whatever imman you want to use,then click on that arrow thing again,then close the window,there you go. Whenever you feel mana and life hungry,you can just eat both at the same time.
I'd explain you how using 2 different eating slots work but by the time you do this you'll have it figured out yourself.