Instructions
Enter Turnback Cave from Sendoff Springs. Bring a Pokemon that knows how to use Surf and ride the water to the cave entrance on the opposite shore.
Study each room you enter in Turnback Cave. Each room has a collection of rocks on the floor beside the left, right and northern exits. Go through the door beside the rock pile that does not match the other two. For example, if the left and center exits were decorated with three rocks each while the right exit has six, take the right exit.
Choose the rooms decorated by strange rock patterns with the goal of entering rooms decorated by a single central pillar. Passing through a pillar room means that you picked the correct exit.
Leave the third pillar room to meet Giratina in the middle of the chamber. Save your game. Walk up to the towering monster to battle and capture it.