Here are the steps how to add a new filter to your category and product
1. Go to "Admin area", select the "List Category" and then click "Filter" button for the sub category
2. Click "Manage" button and then "Create New" button to create a new filter to the current category
3. Add something to Key field and then click "Save"
You can create few more attribute for the current filter
4 .After created the new filer, we need associate the filter with a product under the same category.
Click the “Edit” button for a product under the same category, and then click the "Edit Attribute" button
5. And select one or more filter from the drop down and click “Save”