Online application followed by online test. If you are successful you get invited to an assessment centre. It is a half day assessment centre now (approx 8:30 am til around 1pm). It is really quick, I applied mid Sept 17 and had my assessment centre by mid Oct 17 (4 weeks total). You hear back within 2 working days of the outcome. Unfortunately I didnt get an offer, but you get excellent feedback.
Assessment centre involves: motivational presentation (you get the topic beforehand) where you give a 5 min presentation and answer questions in a 1 to 1 environment. Make sure you know about the business and the role. Need is an interview. The interview is quite in depth and looks to see your motivation, your knowledge of the company, what the role involves and quite a few questions where you need to use the STAR approach (i.e. describe a scenario). You also have a short written case study where you are given a problem and have around 20 mins to read it and give your responses. Finally, its a fun group exercise.
I'd say make sure you fully understand what Rolls Royce does and what the role is about. If not you will struggle. But it was a good experience and the assessors are really friendly.