Sale effect on products HTML

I would like to add the effect "% sale" on some products but I don't know how to do it!

To be explicit : I have an object with price below and I want to add an effect like " this product is in sale of 20%" maybe the old price could be red with a "/" on the value with the new price near it and a different color like black or other... It's the classic effect of sale products "old price-sale of x%-new price". If you don't understand ask information below,I'll respond faster than I can,don't press only "-1" thank you!

It's for my web page,not work or other...