Technically they say you don't need to prepare as they're looking at capability.
In practice, see how much you can teach them to read. This is what we saw at Hutchie's assessment - parents practicing reading with their child, while we looked on thinking, "ds knows the letter Mmmmmm the first letter of his name "
He didn't get in.
This was probably just as well, as he wasn't developmentally ready to read until he was 6.5, despite lots of fantastic support from his (state) school, who made sure he wasn't put off reading.
He's just got 6 As (including English ) a B and a C for his Nat 5s (The C, for French, he expected but he was/is inconsolable about his B as it was in one of his favourite subjects, that he's doing for Higher: he'll find out tomorrow what the problem was when he starts back at school).