Nymph fishing under an indicator has become one of the most popular methods used by fly fishermen while fishing for trout. There are several different types of indicators including yarn, foam pinch on, balsa, plastic balls or styrofoam shapes. I prefer the styrofoam ball because I can quickly change depths while fishing. I use a small piece of rubber band to attach the indicator to my leader using the following steps: 1) Select the smallest indicator that will support the weight of your fly and any additional weight such as a split shot. 2.) Fold the leader, twist the fold and insert the fold thru the hole in the indicator. 3.) Untwist the fold after passing thru the indicator and insert a small piece of rubber band thru the loop of leader protruding thru the indicator. 4.) Pull the two ends of the leader on the other side of the indicator drawing the rubber band into the hole of the indicator until the rubber band shows at the bottom of the indicator. Your leader should now be straight with the rubber band attaching it to your indicator. This allows you to slide the indicator up or down your leader to adjust the depth. It also allows you to remove the indicator by removing the rubber band .
Remember to use the smallest indicator possible to support your rig. Also learn to watch the indicator as well as looking below the indicator while fishing shallow clear water. We sometimes depend to much on the indicator to detect a strike. Watch for a flash or the fish taking your fly. Even with an indicator, trout sometimes take the fly by only opening and closing their mouth and may reject it before you see the indicator move. When you see the trout flash below the indicator----set the hook! When in doubt whether it's a strike or not---set the hook! A take can sometimes be very subtle.