I am posting this mainly as a record for myself as at times I do a project say crotchet and then do not do another for some time so forget how I have done something.
Firstly I completed the required granny squares. I then put them right sides together and matched up the holes on each square.
Then placing the hook through the first set of holes started the join. I have done a single row of crotchet along the edge which on the wrong side produces a ridge and the right side a lovely neat finish.