Been through 11+ last year, somehow survived!
general advice I would give:
identify target schools
go to all the open days (take child if possible)
get a feel for the fit / culture/ your own needs
once you know the schools, go on the websites and get all the entry details, test formats
some will be a mix of computer tests and written, some are multiple choice etc
once you establish the different formats, gently begin to prep your child with the relevant materials eg atom, cgp books, even show them some practice questions from sample papers.
use this to identify gaps and focus on them yourself or engage a tutor with experience of your target schools.
starting early is good in a way, you can get into a consistent rhythm of little and often
for particular schools if you know people at them or who have been through the same process, try and lean on them of help
i am only familiar with Sw London schools
hope that helps and best of luck!!