<label for="phone">
  Enter your phone number:<br />
  <small>Format: 123-456-7890</small>
</label>

<input type="tel" id="phone" name="phone" pattern="[0-9]{3}-[0-9]{3}-[0-9]{4}" required />

label {
  display: block;
  font:
    1rem 'Fira Sans',
    sans-serif;
}

input,
label {
  margin: 0.4rem 0;
}