Identify the perspective first -- eg. if it was a government-issued recruitment poster for WWI, then say something along the lines of "Source A's perspective is that of the [country] government during the outbreak of WWI." When accounting for perspectives, you're essentially explaining why the composer of the source has that perspective. Again with the poster example, you would say something along the lines of "As an official body of authority in the midst of conflict, its main objective would be to support the war as much as possible; this would include establishing a strong armed force to fight the enemy. As the government was also looked up in the midst of such crisis by the general populace, it would combine these two factors, culminating with the aim of convincing civilians to join the war effort and enlist, by making it look like an enticing adventure." Something like that. You think about what the composer of the source's motives could be, their position/context etc, and link it to the motive of making that source. When there's more than one source -- where the sources are mostly either presenting seemingly contradictory information or one source has more info than the other -- you would include something in your paragraph along the lines of "While Source A has this, B has this."
Also, don't mention the word 'bias'. Just because a source has one perspective doesn't mean it's biased.