Usually you will get a link to the prep software with your invitation (at least I did).
Once you do, start straight away. These tests are designed to see how much time you are willing to spend preparing yourself.
At the beginning there will be a lot of frustration, but don't let that deter you. After a while things will get better.
To give you one example:
There is a test where a voice says a sequence or random numbers ( 8, 3, 2, 9, 4, ...). At some point the voice will stop, but you don't know when.
Your task is to write down the last 6 (or so) digits, but in reverse order!
The first time I tried my brain was stumbling about and after the test I didn't know if I should be crying or laughing (I went for laughing).
After a few days I am getting pretty good at it. My record so far has been recalling 8 numbers.
So practice, practice, practice.
By the way, which bases did they offer you and which one did you choose as your nbr1?