Step 1: 1) add a bit of icing sugar and around 1 tsp/tbsp of cocoa.
Step 2: 2)put a few drops of water (not too less and not to much) and mix.
Step 3: 3)it should be a bit runny.
Step 4: 4) now melt the butter.
Step 5: 5) add the melted butter to the icing and keep mixing. (if you stop mixing then it will become hard.)
Step 6: 6) spread on top of cake or whatever you are putting it on.