First, catch your pet already relaxing. When they are lying down, say the word "relax" in a low and steady voice while petting them with a long, gentle motion. Repeat this step throughout the day for a few days.
Then ,ask your pet to lie down and gently pet them. When your pet lies down, repeat the cue: “relax.” Do this step a few times over a week until you can ask them to lie down, and they do so quickly. When they do, again say the word "relax." When your pet does this, reward them with praise and pets.
The goal is to train your pet to automatically lie down and chill out when they hear the word "relax," so when they are in stressful environments, like a busy vet's office, they can quickly find their inner peace.