Food for dog. How do I prepare their food?

Ask the owner to provide their usual food, along with the other necessities such as collar with ID tags, leash, well fitting harness,. medications, vet contact info, emergency contact info.,...and then there's a list o optional things you could ask for them to provide including treats, bedding, comfort item with their scent. etc.

If they've boarded previously, they likely wlll have planned on bringing these items, but a friendly reminder is usually appreciated.

If there's any push back (rare) you can let the owners know that changing food or treats can result in tummy troubles (vomiting, diarrhea) and you want the dog's stay to be enjoyable. Plus, many dogs have their own preferences for specific recipe , protein suited to their nutritional needs.