I'm not an expert fitter, but I do remember the first time I ever really watched somebody do a tailhead and understood what was going on. The key is to create the visual appearance of a corner, and the animal isn't exactly built that way, so there's a little bit of an illusion, and you have to understand that before you can make it happen. Think about it like you're trying to make a corner on a circle, because the actual shape of the tailhead has a curve, not a corner.
In very basic terms, I find the highest point of the tailhead coming out of the spine, and I take the hair as short as I can at that point. There will usually be two spots actually...one right on top of the tail head and one a few inches in front of that where most cattle have a little "bump" but DON'T clip the hair between the two spots! From the high spot of the tail back, the hair is brushed back, toward the back of the animal. Next, look at the profile of the calf and find the first spot on the back side of the tail that would touch the side if you put your heifer inside a rectangular box. Clip that holding your clippers upward and moving toward the top of the animal's back, leaving the hair long at the point where the tail rounds off toward her back. Use a rice root brush to brush that straight up, and keep brushing the hair on the top straight back. You should have a corner.
Just a note...I hate it when people completely shave the tail, thinking that making the tail narrow will make the butt look wide...it doesn't. A skinny tail will make your heifer look fine boned or like she's the progeny of a large rat! Clip the hair on the BACK of the tail...leave the sides. You can "fuzz" the sides and make the tail look wider, and it actually "fills in" giving some dimension to the animal when viewed from the rear. Avoid creating a "line" where you lay the clippers down and shave something right beside long hair. Try to blend any area between the shortest hair and the longest hair and the animal will look effortless and naturally well groomed, like she just LOOKS that good all the time!
Go to the big shows and just walk around and watch. Really study the animals that look good, and try to understand WHY they look good. Ask questions when you have a chance...I'm not sure how many people who are really good at it take the time to help somebody who wants to learn, but they might. You can learn a lot by just watching and really paying attention.
Good luck!