Adding Parameters to Existing Django Model Fields

Note: there is another thread on a similar problem, but this question is more detailed and the answer mentioned in the post above is not satisfactory (elaborated below)

I was trying to create a custom Django model field, where the new field will essentially be the same as a CharField but has an additional boolean attribute -- for now, just call it "public."

I have looked up this article, but I don't like the method described as it required me to construct a model from scratch. That solution is a hassle considering the number of functions I must implement to add a parameter, per the docs.

To clarify my objectives, I will like to declare this field as such:

somefield = BoolCharField(public=True, max_length=100, blank=True)
# Note that public is NOT a default CharField attribute

The documentation stated that I can extend new fields from existing ones, but the process isn't elaborated. Can anyone please provide me with an example of how I should implement my field?

This is the source code of CharField in case anybody needs a reference.

Update

I had a look at overriding the default __init__ method. Here's what the documentation suggests:

from django.db import models

class HandField(models.Field):
    def __init__(self, *args, **kwargs):
        kwargs['max_length'] = 104
        super().__init__(*args, **kwargs)

However, the docs didn't specify how exactly overriding works in this case (e.g. where should I throw in an additional attribute and what methods I must include), which made me post this question.

Again, as I have been stuck for quite some time, I'd prefer examples instead of concepts.