How to serialize an extended user?

[AhmadM-DL]

I am building an API based application.
Users register through an API.
For registration an extended user model has been written as proposed by Django documentation:

class UserProfile(models.Model):
    user = models.OneToOneField(User, on_delete= models.CASCADE)
    phone_number = models.CharField(max_length=25, unique=True, validators=[RegexValidator(r"^\+(?:[0-9] ?){6,14}[0-9]$")])
    language_preferences = models.CharField(max_length= 35)

How to write the signup view and serializer for this model?