UPDATE: For a nice and quick tutorial on making cloaks, check out Col. Corbane's Veteran squad cloaks here.
Earlier this year I posted a pic of a couple of Ravenguard character conversions I did as a commission. Quite a few of you were interested in how I did the feathered cloaks. I finally got a chance to throw some putty on a sheet of plasticard and push it around for you. What follows is how I made the feathers for the cloaks on the models above.
Step One: Roll out a thin "sausage" of greenstuff. Lay it down on a flat surface. Ceramic tiles are quite good for this purpose.
Step Two: Use the burnishing (circular, flat-ish) end of your sculpting tool to flatten out your "sausage". You don't want this to be too thick or too thin. Practice will tell you which thickness works best for you. Use plenty of water on your tool.
Step Three: Carefully score a thin line down the center of your flattened greenstuff. Make sure you do not cut all the way through, and that your hobby knife is lubricated (I use my painting water to dip my knife in before scoring). Then start scoring in the "feather vanes" with your wet knife. Keep these small and tight together, and do not cut all the way through.
Step Four: Once you've scored the length of your flattened greenstuff, put it aside to dry completely.
Step Five: When the greenstuff has completely cured you cut start to "cut out" your feathers with your hobby knife. From a strip like the one shown in the pictures above you can cut about four or five. Once you've cut out all of your feathers you can start to glue them to your cloak. Glue them on in layers, starting with the bottom row first and adding feathers so that there's an overlap, the bottom of each row covering the top of the one before.
I hope you find this helpful : )