First you need a keldeo that knows sacred sword. Then you need terrakion, virizion, and cobalion. After that go to Floccessy town to the far north where Alder's house is. Don't enter. Instead look between his house and the fence on the east side to find a gap. Go straight through to find a hidden passage way. Go in it then you'll be at pledge grove. Go straight from there to find a boulder with three markings that keldeo will react to. Once the boulder is reached, and keldeo reacts, keldeo will learn secret sword as well as turning into resolute forme.
