For my yuanji cosplay I used heat n bond
it is an iron-on "glue" you can use to fuse 2 fabrics together. I took reference pictures and blew them up on my computer to the size I needed it to be [measured the skirt to find # of inches the design should be] and used this as a stencil to trace it on to the papery side of the heat n bond. iron 1 side to the darker purple, cut out the butterflies, peel the paper off, then iron onto the dress.
Also dont use the "lite" version, it wont hold as well. you can get it at walmart or joanns that I know of.
Edit: make sure you get the orientation right before you actually cut it out, you might need to mirror the image / flip over your stencils, I forget, but I always triple check before I cut