Second, purchase sheets of candle decorating wax from a place like Nova.
Third, make your design on paper first to get the proportions right. For some designs it helps to make a paper pattern first. A book like this one has some design ideas that we used at Christmastime.
Then, just cut with little cookie cutters or table knives and press into the candle. The warmer you get the wax the better it will stick.
Light and burn as usual. The decorating wax will melt along with the candle.
Here are several sides of one spring candle. The flowers were made from one of these pattern cutters:
Kids to great at this - here is one Sunshine did all by herself!