One of the most common questions folks new to Shopify ask is how to create sub collections. It is actually pretty simple, but requires a little bit of coding.

Let's say you sell Shopify themes, and you want to have sub categories for different styles of theme; say Minimal and Grunge. To do this we would start by creating collections for each of the sub categories. In this case we would have two new collections for "Minimal Themes" and "Grunge Themes".

We then need to associate the the sub-menus with the top-level menu. The easiest way to do this is using a link list (these are created under the Navigation tab in Admin). In this case we would create one called "Shopify Themes", and then we would add links to the two collections.

Now we need to edit the theme so it understands there may be sub-menus. Typically, the code for the menus is in theme.liquid. Open it up and look for the code that is creating the menu items. You need to modify this so it looks for a matching link list, and if it finds one it outputs those links as well. Here is some sample code from the Vanity theme:

{% for link in linklists.main-menu.links %}
<li>
	<a href="{{ link.url| escape }}">{{ link.title | escape }}</a>
	{% if linklists[link.handle].links.size > 0 %}
	<ul>
		{% for sublink in linklists.[link.handle].links %}
		<li><a href="{{ sublink.url }}">{{ sublink.title | escape }}</a></li>
		{% endfor %}
	</ul>
	{% endif %}	
</li>
{% endfor %}

Now your page will have the additional links under the top level item. 

You may need to add some CSS styling to make the menus appear properly. Alternatively, pick an example menu from the excellent CSS play site: http://www.cssplay.co.uk/menus/

That is it. You can see it in action here: http://getvanity.myshopify.com.