I've just been thinking. (Not always a good thing!)
Do we (parents/guardians) apply to sit the 11 plus or is it done through the primary school the child attends?
This depends on the school/region. Check the admissions page(s) of the schools you are interested in or ask in the forum for the region you are in.
IWhen do you start to look at schools after 11 plus or before?
I really don't understand how any of this really works, and need to learn dverything in super quick time
We went to open evenings of various schools in both year 5 and year 6. Going in year 5 gives you time to think about schools and ask questions. Also helps possibly motivate DC once they have seen some schools.
Leaving it until year 6 may mean you have spent several months working towards an exam only to find the school isn't quite right and that another school that you only went to look at 'out of interest' is actually the one for your DC.