Castrol,
The document attached to the email you got describes it pretty well, probably better than I could. As far as preparation, maybe practice a bit of mental arithmetic, just simple stuff. Don't worry if you think you didn't do as well as you thought once you finish. You don't have to get every single one right. Some of the tests give you an opportunity to practice them first. If you've got till next Tuesday, then it's worth doing them over a few days and practicing them more than once. Get yourself into the zone, so to speak.
Keep in mind too, if you get through to the assessment day, you'll be doing some of the tests again. They like to make sure it was actually you that did the test and that you can do them under different circumstances.
Good luck.