How should I prepare for the ELAT?

Oxford and Cambridge stress that the ELAT should test students' skills in analysing unseen literary material, rather than expansive literary knowledge; you cannot revise for it in the same ways you would for an English A level exam (learning quotations, themes, etc.). One of the best ways to prepare, therefore, is by going through practice papers, which can be found here: https://www.admissionstesting.org/for-test-takers/elat/preparing-for-elat/. I would recommend doing the first one in your own time and mulling over how to plan your essay for as long as you need; over the course of a few practice runs, work up to completing an essay within the 90 minute time frame. The three most important factors of the ELAT are: (1) writing a coherent essay within the time limits; (2) analysing your texts really closely, as they are all you have to work with; (3) making sure that your essay is comparative and that you are making frequent, relevant links between your two texts. Try not to write a huge block paragraph on text A and then another for text B, as examiners tend to prefer you to forge comparisons throughout the essay. You may also want to take note of the information given about each text: think about its form and date, how this affects the way is was written and/or the way we read it.
Whilst you may feel a lot of pressure to give yourself the best possible chance of doing well in the ELAT, it is important to remember that everyone is in the same boat (its unseen nature is supposed to create a level playing field). It is only one part of your application: for Cambridge, at least, most applications are filtered after interview, and the quality of your personal statement also contributes to the decision made by the University. Keep practising, remember to think about language, syntax, form, structure, imagery and allusion, and good luck!