Guide: How To Cook (ign.com)

Step 1: Hunt and Gather
Link must have ingredients on hand before he can cook! To make a heart-refilling meal, hunt animals for meat and gather fruit, nuts, mushrooms, and vegetables. Animals are very wary of Link, so you need to be sneaky to get close enough to fire off a deadly arrow or throwing spear. 

If your aim is to make a stat-boosting or resistance-enhancing elixer, collect bugs, critters (like lizards), and monster parts from the enemies you defeat. 

If you don't know whether the material is meant for a recipe or an elixer, just check to see if it grants hearts and read its description. Food grants hearts (generally speaking), elixer ingredients do not. 

Step 2: Find a Cooking Pot and Fire

Try to remember where there are cooking pots for you to use in the The Legend of Zelda: Breath of the Wild's world, especially ones nearby fast-travel points. If the pot's fire isn't lit, you'll either need to light a torch at a nearby fire and bring it to the pot, or place flint below it and strike it with a metal weapon to light it. You can find flint by breaking the glinting rocks jutting out of mountain walls with bombs or weapons. 

If it's raining, and it's uncovered, well... You'll just have to wait! 

Step 3: Cook!

With ingredients gathered and pot hot, it's time to cook! Go to your materials inventory and choose to hold an ingredient. From here, every item you select will automatically be held. Once you're holding everything you want to go into the pot, up to five ingredients, exit the menu and choose the cook option in front of the cooking pot. 

That's it, you've cooked!