Here are some examples:
* Crunch: for biting into a crisp apple or a cracker
* Slurp: for enjoying a bowl of noodles or a milkshake
* Munch: for steadily chewing on something like a sandwich
* Gobble: for eating quickly and enthusiastically
* Chomp: for taking big bites out of something like a steak
* Gulp: for swallowing quickly, like a drink
You can also use onomatopoeia to describe the sound, like "nom nom nom" or "munch munch munch."
Ultimately, the best way to "spell" the sound of eating is to describe it in a way that captures the feeling!