There are two methods I'm aware of (there may well be more out there)
1) for patterning a piece you've already made (let's say a cloak) you can make a green stuff press mould of the cloak and then, while its still a little soft, peel it off the piece and use a knife/sculpting tool to press the shape of your desired filigree into the mould. You can then use it to produce copies of the piece you moulded+filigree.
2) option 2 is just the plain ol' sculpt it carefully. Use wee sausages and press away sections on the edge to thin it out and shape it. You probably have to plan it carefully so you don't find yourself in a situation where the sculpting tool won't fit between lines of filigree. If you need better descriptions, try PMing egotyk and asking how he does it - he used to work for Rackham and is well practiced at the art of wee fiddly bits
