Click on the market icon on your MT4/mt5:
Click on Indicators and you can also use the search bar to type and search the indicator example type "Smart Trend Trading System":
Select the Indicator you want and click buy and follow steps of purchasing using your preferred payment method:
After purchase you will see Install and open Button, typically Install button first after the install you will see the open button:
After install and open you will see the indicator name in the navigator: 
Click and drag the indicator to the desired chart:
you can navigate to inputs to see all the input settings of the indicator:
Also you can navigate to colors where you can change the colors of the indicator, certain colors can be changed in the inputs bar for easier access:
Click OK and the indicator will be applied, now the indicator is ready:
