This is the structure I generally follow:
Written:
Intro: register, social purpose, audience, contextual factors
Syntax
Stylistic features: lexical choices, semantics, morphology and/or orthography
Coherence: how the text is ordered, formatted etc.
Cohesion: how the text flows
Spoken:
Intro: register, social purpose, contextual factors
Phonological features: prosodics, use transcription symbol
Stylistic features: lexical choices, semantics and stylistic devices
Spoken discourse features: non-fluency features etc.
Spoken conversational strategies: adjacency pairs, topic management, turn-taking, floor-holding strategies.
To do well, I recommend going over all of the sections above and learning the features of each. Also remember in the response that after you point out the feature, relate it back to formality/informality as well as the social purpose, and any other necessary elaboration.
I hope this helps!