You have to advance the frame by one.
Simply, if the room you get Giratina in has wild Pokemon, just walk forward one step (without turning) and catch him right away. Make sure you use repel just in case.
If it doesn't, walk 128 steps with ONE Pokemon in your party, then catch him right away. Make sure you hit your target time in delay though by using the coin flip method so you don't catch Giratina a few times and realize he's the wrong one.
Your target frame is 2 right?
EDIT: ixin you need to know its level.