I don't quite understand when to use the perfect tense vs the imperfect, could you help me please? (making the assumption they understand the conjugation rules already)

The imperfect tense is used to express things that 'were' happening or 'used to' happen. They may have taken place over a short period of time (a few seconds), or a prolonged period of time (several years) but have since finished. In French there is only one structure for both the 'were' happening, and 'used to' happen, it is the context that tells you which version you're using. Examples:
I was playing football last Saturday - Je jouais au foot samedi dernierI used to play football when I was younger - Quand j'étais plus jeune je jouais au foot
The perfect tense is more for expressing defined actions that you 'have done' or 'did'. These are one off events and completed actions. Again in spoken French there is only one structure for both the 'have done' and 'did'. Examples:
I played football last Saturday - J'ai joué au foot samedi dernierI have eaten too much cake - J'ai mangé trop de gâteau