Tone

Keep the tone conversational and polite (research supports this approach). Use an informal writing style as if you are speaking to the student one-to-one, but keep sentences short. Avoid colloquialisms and consider your audience; some students may be studying for the first time since leaving school (possibly some time ago) or may not speak English as their first language. Also consider using a disclaimer at the start of the unit if there are images (such as, injuries or wounds) that may cause distress.
For the most part keep the tone in the second person (you). There may be some exceptions, for example the first person may be used in case studies and the third person when quoting or referring to someone else.
Avoid sarcasm as it could be misinterpreted in writing.