Explore real markets back to 2015, try an idea in plain words, and watch every trade it would have made. Then let it run on live prices while you follow along from your phone.
Three steps, and the first takes less than a minute.
Every market is a living line of its real price. Travel back through years of it, zoom into a day, see what moved.
Pick one to start from, or say your own: “buy EURUSD when rsi(close, 14) < 30”. Every trade it would have made appears, and you can ask why it made each one.
Start it on live prices and it keeps trading around the clock. Your phone tells you each time it does.
Genesis is built so what it shows you is what really would have happened.
A test only ever knows what was known at each moment. Peeking ahead isn't a mistake you have to avoid; it's impossible.
Every result says which prices it used, what trading cost, and what it left out. Trying many settings until one looks good is counted against the result.
Change any part of an idea in words and see the difference at once. Keep versions, compare them, and build up a world of your own.