Tell us what you are trying to do? Create two sorting dropdowns, one that filters "type" and another for "sector," which were created using a checkbox item. I would also like this to say "Type" and "Sector" not "Post Type"
Is there any documentation that you are following? No.
Is there a similar example that we can see? hidden link
If you are adding filters for checkboxes fields then by default the filters would display as checkboxes; if your question is how to display as dropdowns, you can change that in the "Field settings" in the sidebar when the filter block is selected.