Кратко, после проверки валидации берешь username и password из формы, делаешь запрос по ним в базу и если такой пользователь с таким паролем существует, применяешь метод login_user() из Flask-login

# -*- coding: utf-8 -*-
from flask import request, render_template, flash, g, session, redirect, url_for
from flask.ext.login import login_user, logout_user, current_user, login_required
from app import db
from app.models import User
from app.forms import LoginForm
    
@app.route('/', methods=['GET', 'POST'])
def index():
    form = LoginForm()
    if form.validate_on_submit():
        username = request.form['username']
        password = request.form['password']
        user = User.query.filter_by(username=username, password=password).first()
        if user is None:
            flash(u'Неверный логин или пароль', 'error')
            return redirect("/")
        login_user(user)
        return redirect("/")
    return render_template("main/main.html", 
        form = form)

@app.route('logout/')
def logout():
    logout_user()
    return redirect("/")

models.py

from app import db

class User(db.Model):
    __tablename__ = 'users'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(100))
    username = db.Column(db.String(120), unique=True)
    password = db.Column(db.String(120))

    def __init__(self, name, username, password):
        self.name = name
        self.username = username
        self.password = password

    def is_authenticated(self):
        return True
	 
    def is_active(self):
        return True

    def is_anonymous(self):
        return False

    def get_id(self):
        return unicode(self.id)

    def __repr__(self):
        return self.name

template

{% with messages = get_flashed_messages() %}
	{% if messages %}
	<div class="flash">
	{% for message in messages %}
	    {{ message }}
	{% endfor %}
	</div>
	{% endif %}
{% endwith %}

<form method="POST" action="/login">
    <div>{{ form.username.label }}: {{ form.username(class="css_class") }}</div>
    {% if form.username.errors %}
        <ul class="errors">{% for error in form.username.errors %}<li>{{ error }}</li>{% endfor %}</ul>
    {% endif %}

    <div>{{ form.password.label }}: {{ form.password() }}</div>
    {% if form.password.errors %}
        <ul class="errors">{% for error in form.password.errors %}<li>{{ error }}</li>{% endfor %}</ul>
    {% endif %}
    <input type="submit" value="Go">
</form>

views.py

from .models import Team
from django.views.generic import ListView

class TeamList(ListView):
    model = Team

template

{% for item in team_list %}
	{{item.name}}<br>
		{% for user in item.team.all %}
			{{user.name}}<br>
		{% endfor %}
{% endfor %}