How do I focus?
A loupe is a jeweler's magnifying glass. The most common is one of 10X magnification, a teardrop shape of metal from which you fold out a thick lens. While the loupe is invaluable to the gemologist in that it allows direct observation of numerous characteristics of a gemstone, its function for the customer, and in particular the emerald buyer, is more simple. Rather than using the loupe to observe the inclusions in the emerald before you, the most important use for the loupe is to look for major blemishes on the surfaces of the stone, and in particular chipped and broken surfaces.
As your experience becomes greater you will learn more about the observation of the inclusions visible under the magnification of a loupe, but to start with all you need to do is to learn to focus and then observe carefully all the surfaces of the stone before you. To focus, bring the loupe up to your eye, and then bring the stone up to within about an inch of the lens. At this distance you can generally brace your hands against one another to cut down trembling or movement, for a better observation.
Once focused, move the stone about in the presence of a good light source until you manage to get a reflection of the light source off of each facet of the stone. When you manage this reflection or mirror-like flash of the light on the surface you will be able to see even the most faint imperfections in the surface.
At a particular price point you may need to accept some faint markings of internal fissures showing up on the surface, but you definitely need not accept stones that have been chipped or broken, nor those with pits and other significant blemishes on the surfaces.
Perhaps the best first lesson for preparing to look at any gemstone, not only an emerald, is to ask for a loupe. You need to confirm that this is a 10X magnifying glass, not the kind for reading text, which is at best 2X or 3X. With your loupe in hand, and with the only objective of examining surfaces, you can save yourself untold grief - not to mention money.
My personal policy is to reject all stones with chips, holes, or significant inclusions on the top surfaces. I will allow somewhat more latitude on the bottom surfaces that are not visible when the stone has been mounted.