Matching the Hatch
Fly fishing is all about imitation. The goal is to trick your target fish that your fly is the actual food source of the fish and not hiding a sharp hook that will put a hole in its mouth. Not many people realize that it is a difficult thing to do in order to achieve the perfect imitation.
In order to match what wildlife is going on around the river, the best thing to do is to start flipping over rocks in the river. Flipping rocks in the river will give you a good look at what nymphs to use. Such as stonefly larva and mayfly larva. Seeing lots of these under rocks could point to a future hatch in the river depending on how big the larva are. When the larva turn into adults and start flying is the time to switch over to a dry fly or emerger.
Matching a dry fly hatch is very important. Size, shape, and color all play a key factor into matching the hatch. If one part of the fly is not right the fish will notice it and not go to the surface in order to feed.
Streamers are very easy to match in the water, especially if the water if flowing pretty clearly. Look around in the water and see if there are any bait fish swimming or crawdads. Sometimes you'll get a glimpse of a leech or other worm. After it rains is a great time to use a san juan worm or other worm patterns.
Matching each hatch will improve your fishing skills tremendously. I recommend getting familiar with the local bug life around where you live and the diet of your target species and how that species hunts for its dinner. This will give you a better understanding on how to display the fly you are using.
Good luck out there.