Preparing a former master to run as slave again
The only officially supported method is to make a new base backup, or:


pg_start_backup() on the new master
rsync from new master to old master
pg_stop_backup()
ensure WAL got copied over and applied
create recovery.conf
start resynced server


The 3rd party tool pg_rewind is intended for that purpose. I haven't worked with it as yet and can't vouch for it in production.