I don't have an affiliation with this site other than I tend to catch some great blog posts. Here's a good article on tying flies where it's broken down into 8 essential points.
When tying fly patterns, it’s very important that you try your best to incorporate several different elements of fly design to increase their effectiveness. No one knows with complete certainty what order or priority trout rank each element of a food source or fly pattern, but most anglers agree that the value or ranking of the elements often change depending on how long a trout has been selectively feeding on a specific food source, at what frequency the specific food source is being eaten, and how diverse or consistent a trout’s diet is at the present moment. The order of the elements that I will talk about in know way ranks the importance of the elements. Instead, fly tiers should look at them together as a whole, and try to include as many as possible or as a check list of the features a fly pattern should have when completed. Doing so, they should find there fly patterns more effective on the water for fooling and catching trout. In this post, I will specifically talk about eight different elements of fly design that fly tiers should pay close attention to when tying fly patterns at the vise.