When it comes to mixed drink preparation, you really can have too much of a good thing. A poor balance between your base alcohol and mixers can make a drink either too strong or too weak -- either way, it's too bad. This is why skilled and experienced mixologists all have one critical thing in common: an understanding of how mixed drink ingredients are measured and how their basic ratios work.
Basic Bar Measurements
Bar recipes don't always work in plain ratios -- they often call for measurements. Some of these measurements, like ounces and teaspoons, are recognizable even if you've never made a cocktail before. Others, like a dash or a pony, may not be as commonplace. The former is two or three drops -- while this may not sound like much, when using strongly-flavored ingredients like bitters, a little goes a long way. The latter is 1 ounce, or 2 tablespoons.
Free Pouring and Precision
There are two ways to mix a drink: by measuring the ingredients one by one, or by free pouring them. Free pouring is a system of estimating the amount of a pour by keeping count of the pour's duration -- usually about three or four seconds for 1 1/2 ounces, using a standard commercial pour spout. This is why some cocktail recipes use "counts" as measurements for their ingredients, like a three-count of vodka or a two-count of orange juice. This method is generally imprecise, though, and if you want to carefully control your ingredient ratios, you should measure your ingredients with measuring spoons or a jigger. This is a small, two-sided measuring tool that comes in sizes frequently used by bartenders -- one side may measure 1 1/2 ounces, while if you flip it over, the other side measures 1/2 ounce.
A basic cocktail is made using three different types of ingredients. The first is the base -- this is the principal alcohol of the drink. The second is the modifying agent, which makes the drink more palatable by diluting the alcoholic taste while complementing the alcohol's natural flavor -- this may include liquids like juice, soda or vermouth. The third is the special flavoring, which has a strong flavoring. One classic model for mixing cocktails is the one-to-two-to-eight ratio, which calls for one part special flavoring, two parts modifying agent and eight parts base.
Adapting to Your Ingredients
Because some alcohols and mixers have stronger flavor profiles than others, you should always experiment with a recipe before serving it to a guest. Even if you are using a classic cocktail recipe and measuring the exact ratios using a jigger or measuring spoons, factors like your alcohol's brand or specific ingredients can impact its flavor. Some gins have a stronger juniper flavor than others, for example, while some tequilas are cut with mixers that compromise their quality. Ultimately, don't take any recipe or ratio as gospel -- try it for yourself with the ingredients you have, and adjust it as need be.