I..... actually don't remember how she made the pants... Either paint or iron-ons... She's told me a few times now, and I always tend to forget. The shirt was an iron-on if I recall. I'd message her personally to ask though, if I were you.
My friend, who made a Gamzee costume but needs to remake her horns made her pants using a stencil (cut round) and fabric paint. They came out fabulously, if a little less wonderfully than Diana's (Hanyaan's) if this saves time and/or money.