Very easy as long as you take care with it, building in layers, bulking as you go, and letting it cure completely before moving to the next stage.
There's two tutorials I'd recommend here on dakka:
frock coats and tricorn hats for pirate orks (just trust me - it makes more sense after you read it) and
Whatwhat's Greenstuff Tips and Tricks.
They'll give you the basics to work with to keep your cloaks looking like they belong on your model. I used them both when making my counts-as Tellion conversion.
