app.py

from flask import Flask, render_template
from flask_wtf import Form
from wtforms import TextField, PasswordField, BooleanField
from wtforms.validators import Required, EqualTo, Email, Length
from flask_bootstrap import Bootstrap


class UserForm(Form):
    username = TextField('Username', [Required()])
    password = PasswordField('Password',
                             [Required(), Length(min=8, message="The password needs to be at least 8 characters long")])
    email = TextField('Email', [Required(), Email()])
    role = BooleanField('Administrator', [Required()])


if __name__ == '__main__':
    app = Flask(__name__)
    app.config['SECRET_KEY'] = 'lame'
    Bootstrap(app)

    @app.route('/')
    def index():
        return render_template('formtest.html', form=UserForm())

    app.run(debug=True)

formtest.html

{% extends "bootstrap/base.html" %}
{% import "bootstrap/wtf.html" as wtf %}

{% block content %}
  {{ wtf.quick_form(form, button_map={'submit_button': 'primary'}) }}
{% endblock %}