Invariably, a major problem a beginner fly tier has is fly proportion. Fly proportion = the amount of and length of materials and where they are placed on hook to form a fly where the individual parts have the correct relationship to each other.
Here is a must read article by Charlie Craven on dry fly and nymph proportions.
http://www.flyfisherman.com/2013/12/...autiful-flies/
Here's a related article on tying hair wings.
http://www.flyfisherman.com/2014/01/...ng-hair-wings/
I suggest saving these articles as PDFs that you can refer to as needed. FF often takes done the articles after they have been up for a while. The above URLS may not work in a few months.