If your school does not offer IB after Grade 10 you will need to apply before hand to schools that have the IB option. Unfortunately you cannot do the IB yourself without being in a school that does it as for the IB exams you need to be in an IB accredited school.

In addition there are 3 core areas that are done by students independently but monitored by trained supervisors - this becomes impossible if you are not in an IB accredited school.