To create a product filter in Shopify, you can either use a built-in feature or an app from the Shopify App Store. Here's an overview of the steps you can follow:

Using Built-in Features:

1. To begin, log in to your Shopify admin dashboard.
2. From the sidebar, go to "Online Store" and then click on "Themes."
3. In the "Current theme" section, click on "Customize" to open the theme editor.
4. In the theme editor, look for the "Sections" tab on the left-hand side and click on it.
5. Depending on your theme, you may have a pre-existing "Collection List" or "Product Grid" section. If not, you may need to add one using the "Add section" button.
6. Once you have the section added, click on it to configure its settings.
7. Check for the option to enable filters or tags for the product grid or collection list, and enable it if available.
8. Save the changes and publish your theme.

This is a comprehensive guide on how to filter multiple collections, including smart collections, in Shopify and offers a discount on them.

Using Shopify Apps:

1. To begin, log in to your Shopify admin dashboard.
2. From the sidebar, go to "Apps" and click on the “recommended app”
3. A pop-up will open. Click on "Visit Shopify App Store" located at the bottom.
4. To find relevant apps, enter "product filter" or "collection filter" in the search bar.
5. Take a look at the available apps and choose the one that best suits your needs.
6. Click on the app to view its details and reviews.
7. To add an app that meets your needs, click on "Add app" and then "Install app" to add it to your store.
8. Follow the installation instructions for the app, which may include configuring settings and integrating it with your theme.
9. After installation and setup, you can customize filter options and placement according to your preferences.

Please note that the availability of built-in features and specific steps may vary depending on your theme and Shopify plan. Additionally, app recommendations may change over time, so it's always a good idea to review the latest options in the Shopify App Store.