Basic Instructor Course
Here's the brief for the BI Course at Cambridge Gliding Club.
The following documents should be useful to you to prepare for the BI course.
The BGA Basic Instructor Programme and Record
Please fill in this document and get your CFI to sign you off as ready for the course. I can recommend Adobe Fill and Sign (part of the free Adobe Acrobat on a PC, available as a separate App for iPad and Android) if you'd like to fill them in electronically rather than by pen.
Preparing for the BGA Basic Instructor Course - notes for coaches and candidates
Good advice which is worth reading.
Mike Fox has produced a video demonstrating the standard instructing patter - it includes the patter you'll need as a BI.
You can find the patter in writing as part of the BGA Instructors' manual. Learning the patter before the course will be very helpful - particularly if you can try it out in a glider or the Sim.