How do you write an essay plan to compare two texts?

There are two main ways in which to plan comparative essays. The first is taking each text as separate sections of the essay, and the second is to integrate talking about the texts throughout the essay. If you're being asked to "compare" or "contrast" the way the individual texts differ or do certain themes or techniques, the second method of integrating analysis and commentary is more appropriate. It allows you to directly interrogate each text against each other. Then, the way of structuring this essay is to think about bits of the the texts that you know share similarities or obvious differences, turning them into paragraphs.
As an example, you're asked to compare how two different poets present the effects of war in their respective poems, one of which is a poem you've never read before. Think about the first poem, about the techniques the poet used to create imagery of war, stylistic devices that create certain sounds (alliteration, onomatopoeia, etc), structure of the poem, and how that shows the poet's opinion on the effects of war. See if you can then find any similarities or differences in the unseen poem. When you then structure your essay, you can plan it to talk about the imagery in both poems simultaneously, then the structure, and so on. Don't forget an introduction, and a conclusion!