I did this for a pretty heart shaped red 3 teir cake
and I covered in fondant then smeared piping gel on then added crushed sugar in 2 hues of color to get a more full deep color. I have done this on ivory buttercream with clear sanding sugar which is a finer grain and texture to the the sugar you might put on cookies unless you happen to use this for that too!
But I digress, I used sugar shack's crusting butter cream recipe which you canm find all over this site and her's. I just put the butter cream on and then applied with my hands the sanding sugar to the sides and tops then let the bc crust around it. I stacked later at venue and took a soft, new, non shedding bristle makeup type brush to the layers and swept away gently the excess before stacking and it covered beautifully and stayed on. I would use the crusting bc as it will harden some and trap more of the sanding sugar without being concerned in over loading your cake in sugar crystals.