I met a representative from EY at a careers event at my university, and got their email address. They were really helpful and answered lots of questions I had during the application process.
The process started with online verbal and non-verbal tests. Practice lots of these and they get easier. Then a competency-based interview, which had the obvious questions (why EY, why consulting) but also had some unusual ones (how do you make friends with new people?) that got me thinking on my feet. Have a couple of examples of things in your background that you could talk about from different perspectives, e.g. a project you managed you can talk about to show your teamwork or time management skills. Also relax and smile, it's a normal person interviewing you, and they're often bored doing dozens of interviews with nervous candidates.
The next step is an AC, which has a group exercise, a psychometric test, and a written report. For the group exercise you're time limited, put a clock (phone/watch) with the time on in the middle of the table, so people have a sense of time. You have to read some content and discuss it. The important thing is not what you say or how much you talk, it is you listening to other people, and keeping track of time. Ask questions when other people are talking, rather than just forcing your opinion on everyone. The written exercise was pretty easy, just condense some text and figures, I used bullet points on Word. Then psychometric test just the same as online - some people were complaining that it was hard, but it really wasn't. Some people had a break between them, but I didn't, so it was a pretty gruelling process. Make sure you're well rested and have had a good breakfast. Drink water to stay sharp too.
Final stage was a partner interview which was much less intense than the AC. My partner was very friendly. The key here is to relax because it's more of a personality fit interview. They'll still ask you the normal questions (why EY, why consulting), so have some polished answers with evidence ready. Try to come across naturally, and go through your background and the job description, talk about how it's a natural fit for you. I got a phone call a few hours after offering me the job.
Just as a further comment, I would recommend applying to similar jobs then it's good to do ACs and interviews for other companies because practice makes perfect, and you'll be less nervous about the outcome of one particular application. But if you do too many, then you'll dilute the quality of your application, and might come across as washed-out in your interviews, or accidentally say 'PwC' in a Deloitte interview. I'd recommend 4-7 applications. I did 10 applications and got 4 offers.