Tying dry fly wings with calf hair can be tricky if you've not done it a lot. Once you get good at tying with it though, you'll see where you have a lot of control over the shape of the wing on your fly. This is great because it can be easy to make a good spent pattern that you can see easily on the water; or you can copy the living insect by the size & slant of the wings, too.
The video below does a great job of showing all of this, as it demonstrates tying a royal wulff dry fly........that's a great pattern & there are a lot of other tricks & techniques in the video. For instance, you can learn how to tie on a stronger, better peacock herl, working with silk floss & dry fly hackle, too. All in all this is a great video to either get started in, or to hone your skills with, tying dry flies. A book with good photos is great, but you can learn so much more & faster by watching a good fly tier work. In my opinion this is a very good instructional video.