In terms of the position in which you place your egg for candling, it really depends on the model of egg candler you buy. With something like the Brinsea Ovascope Egg Viewer or other ovascope egg candler models, the egg should be placed in a vertical position in the slot provided. Other egg candlers also encourage you to position the egg in a vertical way, just like you would when placing it in an egg cup or egg holder. It's usually wise to always place the eggs in the same position, as this will allow you to get used to seeing them in the same way and quickly being able to identify the embryo in the center, as well as being able to tell whether or not it is developing properly.