Add both of the following tags to the page you would like to enable dropdown filters on:
layout_filters
filter dropdowns: [type 1]|[type 2]
*Replace [type] with the names of each dropdown. For example: Category, Topic, or Subject.
Then, on the individual blog posts or questions add the following tag with the name of the filter category:
filter: [type 1] - [type name]
*replace [type 1] with the name you gave the dropdown and [type name] with the type name.
For example, if in the first step you chose Subject, you would have something like this: filter: Subject - Math