I think the key to completing the online assessments is PRACTICE! There are lots of practice assessments available online for free- I used these ones www.shldirect.com/en/practice-tests
I'd recommend finding a quiet environment with good WiFi, getting yourself a calculator, a pen and some paper, and giving a practice assessment a go...
Then you will be able to judge how much more preparation you need to do before you sit the real assessment. If you're anything like me, you might realise that you need to revise some areas- for me, it was Numerical Reasoning.
Although I'm competent in Maths (having studied it at A-Level), I was unfamiliar with the financial context of the questions, and panicked under pressure- so I set myself the task of completing one practice assessment per day.
I wrote down any questions that I struggled to answer during the time, and would later go away and learn how to answer those questions without added time pressures. I also made a little revision sheet for tricky questions.
As I became more confident with the context of the questions, I switched my focus to answering the questions quickly. I found hints and tips on performing fast mental maths on YouTube, and added these to my revision sheet.
After a few weeks, I felt ready to take the real assessment.
It's also worth noting that online assessments are meant to be difficult! Don't be frustrated if you can't answer every question, and don't put too much pressure on yourself (easier said than done, I know!)
Be prepared, practice, and that's all you can do!
I hope that helps answer your question- please let me know if you have any further questions! Good luck!
Freya R. has provided a really helpful and comprehensive answer so not much for us to add. I would just advise you not to pay for any practice online tests or guidance - there are lots of free resources which are just as good if not better. Remember to follow the instructions carefully when taking our tests and try to relax as much as possible.
Good luck and here are a few links which help you:
Thank you for your feedback!