Wordpress Menu Item Descriptions Into Timber Twig Template

I've enabled menu item descriptions in the wordpress menu screen options and added descriptions to a few menu items.

Does anyone know how I get them to display in a Timber Twig template?

And a sub-question on that: I assume before the twig part I'll need to add something to functions.php - whenever I try a solution to something that says 'add this to your functions.php' and I put it into the functions.php file supplied with the Timber Starter Theme, I get errors, presumably I'm pasting it at the wrong point in that file? Where's the safe place to add code in there?

You can access the description with the post_content property.

Here an example of a possible menu build with Twig - in the a-tags you see {{ item.title }} what is the menu label and {{ item.post_content }} what stands for your description:

<nav id="menu" class="wrapper-menu-main">
<ul class="nav-main">
    {% for item in main_menu.get_items %}

        <li class="nav-main-item {{ item.classes | join(' ') }}">

            <a class="nav-main-link" target="_blank" rel="nofollow noreferrer" href="{{ item.get_link }}">{{ item.title }} {{ item.post_content }}</a>

            {% if item.get_children %}
                <ul class="nav-drop">
                {% for child in item.get_children %}
                    <li class="nav-drop-item {{ child.classes | join(' ') }}">
                        <a class="nav-drop-link" target="_blank" rel="nofollow noreferrer" href="{{ child.get_link }}">{{ child.title }}</a>
                    </li>
                {% endfor %}
                </ul>
            {% endif %}
        </li>
    {% endfor %}
</ul>

The main_menu in this line:

{% for item in main_menu.get_items %}

is your twig menu object - it can have a different name in your code of course.

Basically you don't need anything extra in your function.php you just have to get the menu object with twig like:

$context['main_menu'] = new TimberMenu('primary');

Where primary can be a different name in your case depending on how you did registered/named your menu.