Go to the Filters tab and click the plus button.

Enter the filter's title and choose the attribute, which will be used for filtering.

For example, we’ve created a filter called “Dog food” and based it on the product type.

Next, you need to add values (product types in our case) for your filter. Click the plus button.

Add the necessary product types for filtering.

Please don’t forget to click the Add values button after you choose the values.

All values can be edited after you add them.

Note: these values aren't excluded from the standard filters!

Now, we have our own filter “Dog food” on the Search results page.