Was relatively simple during mine (around 2013/14). Online application followed by some standard numeracy & literacy tests.
The telephone interview which followed is essentially to ensure that you're a normal person who can articulate well.
Then follows a recorded video interview. There is no invigilator, but there is a time limit for questions, so it's a good idea to plan your answers and read them out as you would in conversation to test they fit the 1-2 minutes limit. It's also advisable to ensure there's no 'waffle' in them. They want as clear and concise answers as possible, showing that you can think from multiple perspectives.
After this is an assessment day with 5-10 other applicants. Here they essentially want to know that you have a handle on what the role is.
-They'll give you some reports and ask you to pass judgement on what the numbers mean
-They'll show you some product and ask you to decide on which is likely the best seller/most expensive etc
-Then you'll be asked to work within a group.
As long as during all this you remember a Merchandiser's prime directive (to bring in revenue) and portray this confidently, you'll be fine.