Firstly Paint your four fingers in the sprinkle colours you have chosen. I would suggest bright and fun colours. Ive gone for Barry M - Red. LA colour - Purple. OPI - Orange and Barry M -Pink.
Then on your thumb do a square shape in the darker colour of your choice of cupcake colour, i have chosen Barry M - Dark purple. Then with the white do like a splodgey white blob, then indent into the dark purple with the white and a paintbrush to make like little waves. Then using the light colour of your choice of cupcake colour where you have made the waves draw vertical lines, dont worry if you mess up and get the colour on the white you can go over it with the white again.
Then what i chose to do was dots and stripes on the other nails to keep it simple and cute. You can do the dots with either a nail dotter/toothpick/hair bobby pin and the stripes small thin paint brush.
Then using your base colours on your fingers, use either a nail dotter/toothpick/hair bobby pin and make sprinkles and a cherry. Then wahlaahhhhhh!! Cupcake nails ...
Need any more help just leave a comment!