To download and install indicators from MQ15, all you have to do is find the indicator from https://www.mql5.com/en/market/mt4/indicator and select Download. You can then open the indicator in MetaTrader. To install indicators from other sources, save the indicator where you can easily locate it. On MT4, navigate to File > Open Data Folder > MQL4> Indicators. Copy the custom file to the Indicators folder. Restart the program.