Firstly, make sure you know exactly what these terms are refering too – broadly speaking, tonality refers to use of keys and modes, while harmony refers more specifically to use of particular chord successions and progressions, and particular intervals. These definitions bear repeating, as it is vital to stay on topic in A-level questions and not accidentally start writing about something not strictly relevant to harmony and tonality.
When approaching the question, I would advise starting with the simplest aspects. Identify whether the piece is tonal or atonal (i.e. can it be said to be in a particular key/keys? If so it is tonal). Most pieces will be tonal, in which case you can then identify which keys or tonal centers are used, and potentially also how they are related to each other (e.g. if G major was followed by D major, this latter key would be the dominant). Exam mark schemes will usually also award marks for being specific about where modulations (changes of key) occur, so be sure to include bar numbers.
After you have done this, go into specifics – identify particular chords of interest, such as dominant sevenths, diminished sevenths and neapolitan sixths. If there is an emphasis on a particular type of harmony, such as dominant harmony, mention this. If any pedal notes are used, identify these and give specific information – is it a bass pedal (lowest note in texture), inner pedal (somewhere between lowest and highest notes) or inverted pedal (highest note)? Is it a tonic pedal, dominant pedal, or something else? As with tonality, it is usually best to include bar numbers for each feature you identify.
All these features can be complicated, and one of the most useful things to do with your tutor/teacher is just go over each feature in detail until you understand it well and are able to apply this in practice questions. This will stand you in good stead for describing harmony and tonality in both set work questions and analysis of unseen material.