You'd use Flask-WTF for forms.

https://flask-wtf.readthedocs.org/en/latest/

It can also handle file uploads.

https://flask-wtf.readthedocs.org/en/latest/form.html

You'd need to define a model which describes the form fields, handle it in your view, and add it to your page template.

forms.py

    from flask_wtf import Form
    from wtforms import TextField, PasswordField
    from wtforms.validators import DataRequired
    
    class LoginForm(Form):
        email = TextField('Email', validators=[DataRequired()])
        password = PasswordField('Password', validators=[DataRequired()])

views.py

    from forms import LoginForm
    
    @app.route('/login', methods=('GET', 'POST'))
    def login():
        form = LoginForm()
        if form.validate_on_submit():
            email = form.data.get('email', None)
            password = form.data.get('password', None)
            do_login_stuff(email, password)
        else:
            return render_template('login_page.html', form=form)

login_page.html

    {% extends "base.html" %}

    {% block content %}
    <h3>Log in:</h3>
    <form method="POST" action="{{ url_for('login') }}">
      {{ form.hidden_tag() }}
      {{ form.email.label }} {{ form.email }}
      {{ form.password.label }} {{ form.password }}
      <input type="submit" value="Log in">
    </form>
    {% endblock %}