With a personal strategy, you act along clear guidelines and within a defined corridor. Every single decision must necessarily fit your defined principles. Every trading setup, i.e. every entry and every exit from a trade, must be done according to your strategy. Day trading demo accounts offer one way to comprehensively test a strategy. Many brokers offer these.
Once you have developed an appropriate day trading strategy for yourself, stick to it strictly. That doesn't mean that change is impossible. Quite the opposite: If market conditions or personal requirements change (e.g. suddenly more time), you can adapt your trading strategy.