What's your advice for approaching source questions in a GCSE history paper?

This really depends on the question; for example if it is asking you if Source C supports the evidence given in Source A you must make sure you focus throughout on comparing and contrasting the evidence given rather than just delivering individual analyses. If you are asked how useful a particular source is as evidence of something, it is particularly relevant to address the the nature of the source (e.g a diary entry, newspaper article, letter etc), the purpose of the source (e.g were there political motivations?), and whether these things affect the validity of the source. For example, it is relevant to take note of whether an individual was present at a historical event that they are describing in a source, and whether the individual is describing the event in the context of a speech trying to persuade audiences to support a particular cause, as this could affect the content they choose to include.
More generally, make sure you do not just state the obvious but try to provide more nuanced interpretations, include own knowledge where relevant if it contributes to the argument you are building, and make sure to thoroughly address all sources if reference to multiple sources is required; some may seem more useful than others but they have all been included for a reason, so try to deduce what they contribute.