Spread butter on a piece of toast or bread.
Cover very thinly with Vegemite (for the optimum Vegemite sandwich you only need a dab).
Dip your knife in the Vegemite, and scrape up just a bit (it will mix right in with the butter and spread easily). Some people like to "marble" the Vegemite into the butter.
Eat it open-faced and enjoy!
Vegemite Sandwich Recipe: https://whatscookingamerica.net/history/vegemitehistory.htm