The Stops Barrier indicator, as discussed in "Building A Trading Template" by Giorgos Siligardos, is available for download here.
Once downloaded, from within the NinjaTrader Control Center window, select the menu File > Utilities > Import NinjaScript and select the downloaded file.
You can review the indicator's source code by selecting the menu Tools > Edit NinjaScript > Indicator from within the NinjaTrader Control Center window and selecting StopsBarrier indicator.