The following code is from the above Youtube tutorial video. You can copy and paste it into your order confirmation email.
Directly under this line -
{% if line.variant.title != 'Default Title' %}
<span class="order-list__item-variant">{{ line.variant.title }}</span><br/>
{% endif %}
Here is the code to copy and paste:
{% unless line.properties == empty %}
<div class="order-list__item-properties">
{% for property in line.properties %}
{% unless property.last == empty %}
<p><small>{{ property.first }}: {{ property.last }}</small></p>
{% endunless %}
{% endfor %}
</div><br/>
{% endunless %}
To see the result, you will need to make a test order.
You can either deactivate Shopify Payments and activate the Bogus Payments gateway for a short amount of time, or you can make a real order on a product that costs $0.