The double top rule states that the bearish reversal is only confirmed when price breaks and closes below the neckline, the support level formed between the two peaks. A common misconception is that the double top pattern becomes tradable once the second top forms; the truth is, a double top is only confirmed and therefore tradable once the market closes below the support level (neckline).
Most traders enter short on the breakdown close or wait for a retest of the neckline as new resistance, placing stops above the second peak. Measure the distance between the peaks and the neckline, then subtract this distance from the neckline to set the take profit level.