Its the distance between the anus and genitals, For pinkies you need to get a lot of practice in ive just started to get 100 % right out of my past 2 litters.

I had to use pictures which are on another forum which i can link you to if you wish.

I wait until they are 3 days old this stimulates the milk flow of the mother, and if im unsure i wait till nipples start to show then its much eaiser.