Here is the fragment again:
Whenever Rosita makes microwave popcorn.
You correctly identified the fragment as a subordinate clause. Whenever, the subordinate conjunction that begins the clause, is the clue.
To fix this fragment, you could remove the subordinate conjunction, like this:
Rosita makes microwave popcorn.
Or you could use Punctuation Rule 1:
Main Clause + Ø + Subordinate Clause.
The correction would look like this:
The dogs begin to drool
whenever Rosita makes microwave popcorn.
Or you could use Punctuation Rule 2:
Subordinate Clause + , + Main Clause.
And the correction would look like this:
Whenever Rosita makes microwave popcorn,
the dogs begin to drool.