When analysing primary sources there are two keys areas to consider - content and provenance. We'll look at each in turn.
Firstly content is all about what the source says or shows. What is it explicitly showing/telling us? Is there anything you can read between the lines? If it is a cartoon, what political point is the cartoonist aiming to raise? If it is a picture what is it showing? Are there any particular lines that stand out as being important? What is the tone of such a source (ie Does it sound aggressive? Threatening? Emotional?) Can you work out the purpose of this source - is it meant to inform, enterain, persaude etc...?
Hence content is all about pulling apart what the source actually says and commenting on that in detail.
Provenance is concerned with authorship. When was it written? Who was it written by? What would be the purpose of such a source? Does it have a particular bias? Who is the intended audience? Is it meant to be public (eg a speech) or private (eg a diary entry)? When and where was it published? How will that affect the source?
When looking at both provenance and context you should apply your own knowledge to challenge the source. For example if it was published in Soviet Russia in the 1950s, we can comment on how it was written under an oppressive regime so the author cannot be critical of the government or face the risk of the gulags.