Not going to give you every bit you need, but follow the pattern.

register.py

@app.route('/register', methods=['GET', 'POST'])
def register():
    form = RegistrationForm(request.form)
    if request.method == 'POST' and form.validate():
        first_name = form.first_name.data
        last_name = form.last_name.data
        email = form.email.data
        username = form.username.data
        password = sha256_crypt.encrypt(str(form.password.data))
        # Add user
        new_user = Users(first_name = form.first_name.data,last_name = form.last_name.data)
        db.session.add(new_user)
        # Commit to DB
        db.session.commit()
        flash('Registered successfully!', 'success')
        return redirect(url_for('login'))

    return render_template('register.html', form=form)