I'm planning that same cosplay. I plan to use Simplicity 4015
view C for the bodice. I've used this pattern several times before, it's pretty simple to follow. The neckline needs to be altered slightly to be more square. This tutorial
shows a shirt being altered to have a square neckline which should give you an idea on how to do it.
Since the outfit is very similar to lolita, I'm going with my experience sewing that. This video tutorial
shows the basics of making lolita skirts. But in this case, don't do the waistband as you'll be attaching it to a bodice to make a dress. The ruffles will be at the top of the skirt and you'll need to make them longer, measure to figure out how long the ruffles should be. The bottom ruffle should be attached at a higher point than where the top ruffle ends. Also don't bother with the lace, as this dress doesn't seem to have any, but do hem your ruffle pieces before gathering them.
Finally here's a tutorial for the choker
. For fabric suggestion, I'd go with regular cotton.