How should I structure my essay?

It's important to remember that Oxbridge aren't looking for anything perfect. Whilst they want to know you can configure thoughts coherently and make connections thematically in an interesting and original way, they know that this is the first time you'll be being asked to write without restriction which can be really daunting. When you've chosen your two texts, my best advice would be to create a key thread that you can weave throughout your essay. For example, if the common theme were to be parent/child relationships, your essay could be structured around loss, or if it were to be clothing, your essay could be structured around the relationship between the interior and exterior. From that point, you have your unique perspective, and you can analyse one or two ways in which the texts are similar in how they portray loss, for example, and then one or two contrasting points. They want to see that you've thought deeply and equally about every side, whilst also having the confidence to stick with one key, governing angle.