Don't let this task daunt you, it isn't that hard, just do a little at a time, over a period of several days, until you have the effect you desire.  The Breed Standard reads that the tail should just reach the dogs hocks,  and though some dogs will have a slightly longer tail, your goal is to trim the tail to that length as closely as possible, and still not have the tail look noticeably sculptured. The Goldens' tail has a rounder, fuller look when groomed, and the triangular wedge seen on an Irish Setter with a "break" at the base of the tail, is NOT the look desired.  The area at the base of the tail is customarily not groomed to separate the tail from the feathering on the butt, as is done with the Setters in particular for the show ring, although on a heavily feathered dog, some shortening of the length can be done to keep the dog cleaner.   
 

Using the thinning shears, start trimming just past that point, giving yourself some leeway, especially if you are a beginner.  
 
 

  Lift the tail and begin thinning, rounding the length of the tail feathering slightly as you go toward the base.  The dog in the illustration had his tail trimmed not that many weeks prior to the photo, so you need to look closely to see that I am trimming about 1" of feathering off.  On the majority of dogs, especially if having never had their tails trimmed before, you will have quite a ragged edge of sometimes quite lengthy feathering, often much longer at the tip, and also very long at the point in this photo that my thinning shear is just beginning to come to.  I make the initial cut of length this way, then come back, moving side to side across the tail breadth, making light and fleeting cuts to blend up the sides of the tail, so blunt cuts are not glaring at you. More extreme length of the feathering at the tails' base, should be trimmed a little also, this particular dog does not have an abundance of tail feathering or coat at this time.  

The final two photos below, show the finished tail. I also do some rounding and blending very carefully at the very tip of the tail, so that when the tail is hanging naturally down, it doesn't look like someone cut it straight off with a blunt knife.  

ANOTHER SIMPLE WAY to groom the length of the tail is to twirl, or wind the feathering of the last 5 or 6 inches of the tail around the tail tightly toward the tip, then hold it tightly with one hand. Bring your thumb and forefinger to tip of tail still holding the wound feathering tightly, so that you can feel the end bone of the tail. Use your thinning sheers to cut the feathering straight off, within about 1" of the tip of the bone/end of tail. Without letting go of the trimmed tail, but dropping your hold down about an inch or slightly more, begin using the thinners to trim the tail so that it looks like a half of a pom-pom, by angling cuts from the sides to the center, leaving the pom-pom a little "higher" in the center, i.e. a nice ROUND shaped tip of tail. When you have accomplished that, you may drop and shake out the tail and it is unlikely that any further trimming will be necessary.

On some dogs, similar thinning at the base of neck at the withers is also necessary. 
Note: topcoat is NOT removed in any of these area's, we are just thinning undercoat to remove some bulk. 
Over zealous grooming in these areas, whether done with thinning shears, or a stripping knife will not preserve the natural look of the dog, nor will it enhance the dog or cover up lessor qualities.  The dog should not have an "hourglass" look when looking straight at the front when done grooming.