Create empty migration:

python makemigrations --empty main

Where main is your app name.

Add operation in generated code. For example, let's change Tip model from main app:

from django.db import migrations
from django.utils.text import slugify

def slugify_title(apps, schema_editor):
    Tip = apps.get_model("main", "Tip")
    for t in Tip.objects.all():
        t.slug = slugify(t.title)

class Migration(migrations.Migration):

    dependencies = [
        ('main', '0008_tip_slug'),

    operations = [

An important thing here that the signals and overridden save method of models that you use here will not be called. If you need it to be called, please create django command instead.