I use a flashlight wrapped in aluminum foil with a pencil sized hole in the middle. Other people use those penlights that doctors use to look in your eye and constrict your pupil. Obviously, don't spin or turn the egg like you would with birds. And i've heard that candling too much can be dangerous and stressful for the developing embryo, so i'm trying to keep it something like once a week.
you can use an ordinary light and see if the egg had a pink or a rose colour and veins are visible then it is fertile if it was dull orange and no veins are in there then it is infertile ( candiling method is used for knowing if the eggs are fertile or not but i dont recommend candling)