class UserProfile(models.Model):
    user = models.OneToOneField(User)
    
    avatar = models.ImageField('Фотография', upload_to = 'users/', null=True)
    description = models.CharField('О себе', max_length = 300, blank = True)
    telephone = models.CharField('Телефон', max_length = 11, blank = True)
    city = models.ManyToManyField(City)
    street_house = models.CharField('Улица, дом', max_length = 35, blank = True)
    flat = models.PositiveIntegerField('Квартира', null=True)
    
    def __unicode__(self):
        return self.user

class Manufacturer(models.Model):
    # ...

class Car(models.Model):
    manufacturer = models.ForeignKey(Manufacturer)
    # ...

#in forms.py
class UserForm(forms.ModelForm):
    class Meta:
        model = User
        fields = ["username", "email"]

class UserProfileForm(forms.ModelForm):
    class Meta:
        model = UserProfile

#in views.py
def add_user(request):
    ...
    if request.method == "POST":
        uform = UserForm(data = request.POST)
        pform = UserProfileForm(data = request.POST)
        if uform.is_valid() and pform.is_valid():
            user = uform.save()
            profile = pform.save(commit = False)
            profile.user = user
            profile.save()
            ....
    ...

#in template
<form method="post">
    {{ uform.as_p }}
    {{ pform.as_p }}
    <input type="submit" ...>
</form>