When I claimed my SP, I phoned the International Pension Centre, 4 months prior to my data of eligibility, which for me was 4 months prior to my 66th birthday.
Best to do it early IMHO in case of issues/delays. Also best to ring them since less info is required as opposed to the postal application form and you do not need to supply any supporting documents which may be required to support a postal application. Took me approx 20-30 minutes on the phone plus about 5 minutes waiting in the queue.