digitalLibrarian did some symbol sets that you can easily print onto some iron on/transfer paper for troll symbols. Alternatively, there’s also a complete tutorial here on symbol shirts as well.
A lot of people asked for the process on foam robotics, so here ya go. Click through for captions.
(via mstrueimage)
I usually use quilting template plastic for stencils myself (so they’re reusable), and I just masking tape them down!
A colored skirt would be fine, I think! There’s so much canon and fanon in this fandom, that taking a pretty generous artistic license is easy!
A dark colored wig would be fine to start with. You can actually paint the fibers, let it dry completely, then brush it back out. It can turn out a bit stiff but that’s not exactly an un-wanted side effect. Don’t rush right into doing to whole wig, though! Trim off a bit, make a quick test weft, and test it out first to make sure your paint won’t melt the fibers/the effect is what you want, etc.
It’s essentially just a tube? A super-pronounced piping, I guess would be the easiest way to describe how best to make it.
Ultimately, the real challenge to making it is the stripes. You could make a fabric on Spoonflower with the stripes at a 45% angle, that way you could still cut it on the bias while maintaining the stripes.
Fabric paint stays longer, in my experience. Just throw it in the drying after the paint is completely dry (give it 24 hours to be safe), and the the heat will set the paint pretty permanently into the fabric.
It varies? For one, depending on WHO you make and then from there the types of fabrics you use, etc. As far as comparable costumes, the closest I’ve made as far as detailing is this costume of Polka’s from Eternal Sonata:

Which weirdly I have no photos of. :| (There’s some of the dress part here, though)
It ran me about $260 overall, not including stockings, wig and shoes.
They can work great with extra care! I recommend a flour paste glue. Use several layers, and be sure to sand it down well! Use very small strips of paper, or even a paper pulp! Surprisingly, toilet paper creates a really effective pulp as papier mache.
Boil 5 cups of water in a saucepan.
In a bowl, mix 1/4 cup of sieved flour with a cup of cold water. Mix to a smooth consistency (a bit like making custard). When completely free of lumps, add the mixture to the water in the saucepan. Gently boil, stirring constantly for two or three minutes until the mixture thickens.
Honestly, I think it’s all up to personal preference. I wouldn’t suggest going for the solid colors that some places sell since then it makes your eyes stand out a bit too much? But if you look for something that at least moderately blends into an eye like the Bella Violet does, you should be fine? Otherwise if you’re still worried, you could definitely go for just some regular violet contacts from a brand on 1-800-contacts!
Mod #2 edit’d! I think that circle lenses would actually be a pretty cute look for a Rose cosplay! They can make you look a lot younger, which is always a plus unless you actually are ~13 years old.