Golden Retreiver

Trimming the tail is accomplished using Thinning Shears as described and illustrated with photos below.
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.

Brush the tail thoroughly, making sure all traces of tangles or matts are removed. Hold the tail down to the hock and place your thumb at the point where the end of the tail meets the hock to mark the length you will trim the tail to match. Note: in the photo, an imaginary line drawn from my thumb will just meet the top of the dogs hock.
This will be the final length of the tail.

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.
The thinning shears are also used, inserting the blade underneath the coat and "into" or "with" the direction of coat growth as in the red lines shown on the photo. A small amount of trimming is done under the ear on the neck area and under the dogs chin where the two red lines are above.
Following the line of the shoulder, if a heavy "shelf" exists as shown in photo, careful thinning of undercoat in the direction shown by the arrows, and combing out, will blend the coat in and reduce the heavy or "over-loaded" look to shoulders and neck. Groom over a period of several days
and "Stop, look, assess" as we did when doing the ears.

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.