Hi Ericco - I had to do a bunch of these for a job in New Zealand a few months ago. It depends which SHL test they give you, there's a whole range. I took an online personality test, and then had to go to London and take the ability tests.
The latter consisted of 3 different tests. 1 - reading tables and graphs of data, and having written questions to answer about them. 2 - a comprehension test, where you read a paragraph of information and are then asked questions on it. 3. a list of numbers/letters, you then find the corresponding numbers/letters from another list.
All are done on paper, on your own in a room, and you're given a relatively short amount of time to complete them. None of them are particularly complicated, it's more to do with how many you can do in a given amount of time.
SHL should have emailed you a link for their website, where you can take some practice questions which will help.