GP toppings and tails
This method has served me well, I think I learned it from Poul Jorgensen's book years ago, not sure.
1. Select GP feathers of approximate length, and soak in warm water
2. Find a bottle, vial or such with an outside radius of approximate curvature you need. (I don't tie flies larger than #2, and find that plastic prescription vials in various sizes work fine, larger for wing toppings, smaller for tails- i usually do a bunch at a time)
3. carefully lay out the wet feathers on the side of the bottle/vial, and allow to dry.
When dry, the feathers will be a perfect copy of the radius of the vial. Then hold the crest in position on the fly, locate the tie-in point, and firmly nick the stem with your thumbnail. I usually then flatten the stem up to that point with small duckbill pliers. If further adjustment of the curvature is needed, gentle thumbnail nicks and a bit of persuasion will usually do it. Then tie 'er in,,,
Good Luck, Hope this helps!