What is the best way to approach analysing a historical source?

Firstly it is important to cover the who, what, when and where of the source to gain a better understanding. Next, the content of the source should be picked apart to find the true point of it. This should help you begin to form an answer to whatever type of question you have been given.  All of these things will affect the bias of the source and its content; this is where you begin to come onto the 'why' and a good analysis of the 'why' can give you a much more advanced answer. 

It is important to remember not to just state that the author of the source is biased, almost all sources are to an extent biased. Instead, use all of the information you have gathered about the source to discuss how this, potentially biased source, can aid your understanding of the period or topic you are studying.