Backtesting is the review of past events with your current trading strategies and plans to ascertain if they could truly work or not. Traders backtest with their indicators and other devised trading approaches directly from the trading platform. This could be done manually or automatically depending on the provision on the trading platform. For this, you would have to learn from your broker's/exchange's platform what they make available for you.

#Warning: Backtesting doesn't often work for indicators, they will trick you, forwardtesting is better in this regard. But for strategies like Price action, Trendline diagram, Fibonacci retracement and breakout and Elliot wave theory, it will work, but they have to be done manually to be accurate.