Emily started carving letters out of plaster, then figured out how to make molds to produce them in quantity. You can see the finished letters in her Etsy shop Kuber. On her blog she has posts showing how she makes the molds — some successful, some not.
Emily makes her molds using a system called pinkysil, which might only be available in Australia, where she lives. There’s a good blog post here about how to use it. Seems like something to file away under “useful to know about for some future project I haven’t even dreamed up yet…”