Before you start, take off any old nail polish with remover. You can apply a base coat, but I didn't because I don't have any.
Paint your nails black. Sorry about the awful photo!
Using a white nail art pen, draw a triangle on your middle fingernail.
Draw a diagonal line on your ring finger and pinkie finger. Draw another line from the bottom left corner of the triangle to the end of your nail.
With nail art pens, a Striper or even a toothpick, draw rainbow lines on your index finger.
Add a top coat!
Et voila! Pink Floyd inspired nails!