Set SECKET_KEY, then flashing message in view function:

from flask import Flask, flash, render_template

app = Flask(__name__)
app.secret_key = 'some_secret'

@app.route('/')
def index():
    flash('Hello, I'm a message.')
    return render_template('index.html')

Then render the messages in layout.html (which the index.html extended from):

{% with messages = get_flashed_messages() %}
  {% if messages %}
    <ul class=flashes>
    {% for message in messages %}
      <li>{{ message }}</li>
    {% endfor %}
    </ul>
  {% endif %}
{% endwith %}
{% block body %}{% endblock %}

Set second argument when use flash() in view function:

flash('Something was wrong!', 'error')

In the template, set with_categories=true in get_flashed_messages(), then you get a list of tuples in the form of (message, category), so you can use category as a HTML class.

{% with messages = get_flashed_messages(with_categories=true) %}
  {% if messages %}
    <ul class=flashes>
    {% for category, message in messages %}
      <li class="{{ category }}">{{ message }}</li>
    {% endfor %}
    </ul>
  {% endif %}
{% endwith %}