Drawing using Procreate Pocket on iPhone. Final product: https://www.instagram.com/p/BvYZTCsFQ00/
A quick note, since my sticker has a white border- it is a tad off (this was warned to me by the printers in advance) though to me it's not a big deal and forgivable since I'm giving these stickers out for free. If borders aren't necessary for the design, it's best to do without it!