To create a filter using Collection please go through the following steps:
Go to your Shopify Admin Store > App > Searchly.
Click on Filters.
Select Manage Filters.
Click Add New Filter.
Add Filter Title, select Create Filter Using & select the option from the drop down, select where you want to show the filter ( Search & Collection Pages in this case) from Display On drop down.
Fill all the required fields and click the Save button.
Run data sync by clicking Data Sync.