Handmade by me!!!
So, you start with a blank white disk of adhesive-backed rubber:
You get out the appropriate blades - I remembered a little bit about doing lino cutting in school, and I started with the smallest blade to carve the detail, and then used the wider blade to remove the background:
I then decided to cut the stamp down so there was less background, and I mounted it onto an old acrylic block to test it with black ink. This allowed me to decide which bits needed to be carved out a bit more: