How To Change Hitstun In Project M?

[Flandre Scarlet]

Repost from the help forum since I had no luck there. How would I go about changing the hitstun in P:M? I am currently using the hackless version but could find the homebrew version somewhere if necessary.
Post Merge: June 26, 2016, 06:43:36 PM
To clarify I already have a hitstun changing GCT code but don't know anything about how to use GCT codes or how to add them to the Project M codeset.