Here are the ways to quickly rehydrate your dog when they are dehydrated:

Oral rehydration salt. If the dog is only slightly dehydrated, you can feed a small amount of salt-containing glucose water. If the dog cannot drink it actively, you can use a needle-free syringe to slowly inject it into its mouth, about 10 ml each time, and feed it every 2 hours.

Feeding drugs. If the dog's dehydration is caused by diarrhea or vomiting, it can be treated by feeding anti-inflammatory drugs or probiotics. After the diarrhea improves, feed plenty of drinking water.

Intravenous injection. If the dog is severely dehydrated, it needs to be taken to a pet hospital to have an intravenous needle inserted. Under the guidance of the doctor, the dog must learn the methods and precautions for intravenous rehydration.