Think 3 main hoops to jump through on 3 separate days over a 3 month period. The first stage is an online virtual aptitude/IQ test. First comes the numerical one, if you are successful with that, they invite you to take the verbal test. I would suggest you search online for practice tests, this could be a purely personal thing, but the numerical test was the most demanding. Revise fractions, percentages, ratios & interpreting data from graphs; do them over & over again until you become quick at them - time pressure makes this test difficult.
If you are successful at stage 1, they will invite you to the offices you have applied to for a first interview. The HR people at Deloitte were, without exception, all very friendly and accommodating. There was a good atmosphere, conducive to me relaxing and being able to express myself freely. The first 1:1 interview was with a line manager from the service line you apply to. Deloitte publish on their website 6 competencies that they look for at this stage. I found prior preparation to be the key here; sit down for half an hour and brainstorm 2/3 examples for each competency.
Stage 2 is the toughie, an all-day affair. You get there early morning and are taken to a computer to sit an e-tray exercise on a fictitious business case. Keep your head, open & read all attachments & answer all the email questions (24) in 60 mins. Next you get 1 hr. to compose a written response to the case. Structure the mail carefully, because after lunch you will face questions on what you write in your final partner interview. I used bullet points. Act professionally with the partner, be prepared to explain weaknesses on CV