How you use them depends on what you want to do. What you want to do also will be a factor is what type you get. There are 'regular' thinning shears and there are finishing thinning shears. The less teeth the more hair they will cut. You can get them anywhere from 24 teeth to 48 teeth.
If you are wanting to thin out the coat you will want one with about 44 teeth. To thin you need to insert the shears into the coat next to the skin. Be sure the tip of the shears is not showing or it will leave a mark. Do a cut, take out the shears and comb out the trimmed out coat. Repeat as needed.
If you are wanting to use them as a finishing shear then you need to get on with more teeth so you will take off a little at a time. Finishing the coat is to get the straglies or longer hairs blended. Say you are doing a rear leg. You will use the shears in a vertical motion from the hip toward the toes taking smoothing the coat.
As far as shears the 44/20 is a good all purpose shear. I'd get a shear with anywhere from 42 to 46 teeth. Those are good all purpose shears.
