5 cups icing sugar
5 tbsp water
That's it! Mix, separate evenly into bowls for however many colours you want, add your colour, mix well, and place half of the contents of each bowl in a separate piping bag fitted with a small round tip (size 1 or 2). I didn't have any so used a ziplock sandwich bag with a tiny corner cut off (after i spooned the icing in).
Water down the remaining icing in the bowls (not too much... just enough so it flows, probably a bit like honey). Spoon carefully into the centre of the biscuits and spread to the edges using a toothpick. The edges should be fairly dry before you do this - it was a warm night in Perth when i did it so was able to start the fill immediately after piping all the biscuits.
If you're doing designs like feathering (you can see a few above), do this while the fill icing is still wet. For designs on top of the icing, wait an hour or two until the top of the icing on the biscuits has set, then use what's left in the bags.