First of all, have you seen our Guide to Candidates? If not, it's here. It gives a good overview of what's expected for the course.
The other resource you might find useful was a letter that went to Wg Shooting Officers and OC SATTs about 18 months ago, here. The document outlines the sort of thing we recommend you get involved with before the course.
Predominantly, the main focus is contact time with the rifle. We expect thorough knowledge of the weapon, so that you can just focus on the Method of Instruction elements of the course, without having to focus on learning anything about the rifle itself (this is the same reason we ask you to try and memorise a few paragraphs in the Guide to Candidates).
You should also get maximum live fire exposure to the rifle, as well as observe some Initial Weapon Training (IWT) weekends. For the IWTs, try and get an experienced SAAI/WI, who's up to speed on the latest approach, to talk through what's going on in the lessons as you observe them. Ideally, follow the lesson in the pamphlet as it's being taught in front of you, so you can start to see how the content translates into practical application.
That's a decent overview of pre-course, but I'm sure other people can share how they prepared, or in hindsight what they wish they had done.
Any other questions, feel free to drop our Adjutant an email direct before applying.
Hopefully see you later in the year!