Кастомная user-модель

# accounts/view.py
from .form import SignUpForm
from django.shortcuts import render
from django.shortcuts import redirect

from django.contrib.auth import login as django_login
from django.contrib.auth import authenticate
from django.contrib.auth.decorators import login_required
from django.views.decorators.cache import never_cache
from django.views.decorators.csrf import csrf_protect
from django.views.decorators.debug import sensitive_post_parameters

@csrf_protect
def signup(request, template_name='accounts/signup.html'):
    if request.method == 'POST':
        form = SignUpForm(request.POST)
        if form.is_valid():
            form.save()
            username = form.cleaned_data.get('username')
            password = form.cleaned_data.get('password1')
            user = authenticate(username=username, password=password)
            django_login(request, user)

            return redirect('home')

    else:
        form = SignUpForm()

    return render(request, template_name, {'form': form})

# accounts/form.py

from django import forms
from django.contrib.auth.forms import AuthenticationForm
from django.contrib.auth.forms import UserCreationForm
from django.contrib.auth.models import AbstractUser
from django.contrib.auth.models import User

class SignUpForm(UserCreationForm):
    last_name = forms.CharField(label='Фамилия', max_length=30, required=True,
        widget=forms.TextInput(attrs={'class':'form-input','type':'text','placeholder': 'Иванов'}))

    first_name = forms.CharField(label='Имя', max_length=30, required=True,
        widget=forms.TextInput(attrs={'class':'form-input','type':'text','placeholder': 'Иван'}))

    patronymic = forms.CharField(label='Отчество', max_length=30, required=True,
        widget=forms.TextInput(attrs={'class':'form-input','type':'text','placeholder': 'Иванович'}))

    email = forms.EmailField(label='Электронная почта', max_length=254,
        widget=forms.TextInput(attrs={'class':'form-input','type':'text','placeholder': 'example@example.ru'}))

    birth_day = forms.CharField(label='Дата рождения', max_length=10, required=True,
        widget=forms.DateInput(attrs={'type': 'date'}))

    inn_id = forms.CharField(label='ИНН', max_length=12, required=True,
        widget=forms.TextInput(attrs={'class':'form-input','type':'text','placeholder': 'ИНН состоит из 12 цифр'}))

    passport_s = forms.CharField(label='Серия пасспорта', max_length=4, required=True,
        widget=forms.TextInput(attrs={'class':'form-input','type':'text','placeholder': 'Серия пасспорта состоит из 4 цифр'}))

    passport_n = forms.CharField(label='Номер пасспорта', max_length=6, required=True,
        widget=forms.TextInput(attrs={'class':'form-input','type':'text','placeholder': 'Номер пасспорта состоит из 6 цифр'}))


# accounts/model.py
# Как сохранить нижепреведённых полей модели?
from django.db import models

class CustomUser(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE)
    email = models.EmailField(max_length=255, unique=True)
    first_name = models.CharField(max_length=50, blank=True)
    last_name = models.CharField(max_length=50, blank=True)
    patronymic = models.CharField(max_length=50, blank=True)
    birth_day = models.DateTimeField()

    inn_id = models.IntegerField(default=0, blank=True)
    passport_n = models.IntegerField(default=0, blank=True)
    passport_s = models.IntegerField(default=0, blank=True)


# accounts/templates/accounts/signup.html

{% extends 'form.html' %}

{% block content %}
<div class="container">
  <form method="post" action="{% url 'signup' %}">
    <div class="row">

    <h4>Регистрация</h4>
    {% csrf_token %}
    {% for field in form %}
      <p>
        {{ field.label_tag }}<br>
        {{ field }}
        {% for error in field.errors %}
          <p style="color: red">{{ error }}</p>
        {% endfor %}
      </p>
    {% endfor %}
    <button type="submit" class="button-submit">Регистрация <i class="fa fa-check"></i></button>
  </form>
</div>
{% endblock %}