1) Add the script to your chart and click the gear to open the settings
2) Select the source (Close, Open, High, Low, etc.)
3) Select the length (default: 14)
4) Select the %K and %D Periods (default: 3)
5) Select whether to plot the overbought/oversold K%/D% crosses, and their respective levels
6) If you wish, set alerts for when these crossings occur
Any questions, feel free to ask. Enjoy!