This should work just fine:
# sudo mkdir /usr/local/var/postgres
# sudo chmod 775 /usr/local/var/postgres
# sudo chown construct /usr/local/var/postgres
# initdb /usr/local/var/postgres
use your username in place of construct. So, if your computer username is WDurant, the code will be:
# sudo chown $(whoami) /usr/local/var/postgres