Right click on chart, find "studies", click on that, scroll down to slow stochastic, highlight, click add, then click settings, when the box comes up, change the parameters as you like, when you're done, click OK, and then click OK again. Now you're done.
I changed the moving average setting to 1 (as you need to if you need the 'original' rsi). I'm trying to replicate the QCharts setting,so if it does this,I'm happy.