I'll be using buttercream, does anyone know how to achieve this effect??

I have used a technique that I found in a Collette Peters book to mimic a knit sweater. It's on the snowman cake in my photos. Using a #3 tip, you just pipe overlapping lines of curlicues. If you search for "Collette Peters sweater cake", I bet you can find a photo and maybe a tutorial.

Hope that helps!