Cut out the cactus outline in green heat transfer vinyl. This will be laid down as the first layer. The one pictured is sized at 10″ tall.
Cut the spines out of black HTV. My favorite is the easy weed heat transfer vinyl for small details like these tiny cactus spines.
Using an iron or heat press center the cactus cutout and press for half the suggested time, carefully remove the plastic film.
Lay the spine cutout over the pressed cactus. Press for the full time.