Firstly, go to the collection you wish to add the sub collection to and when in the SubCollection section, select create new SubCollection.
After doing so, you will firstly see the properties window appear where you should name and describe the SubCollection that you are creating.
The search configuration is the last stage of the SubCollection wizard. Again, it follows the steps of the original search creation step of the main collection wizard.
Searches are very similar to lists in their configuration. You simply start by dragging across the field you wish to search against.
You have numerous operators to control the filter, such as; contains, equal to or has a value of.
You can add in a row beneath that forces the search to look at both, or more criteria.
Search configuration is demonstrated here: