W=Hex attack deter
X=Hex defense deter
Y=Hex speed deter
Z=Hex special deter
You put the code in, turn the GS switch off, turn the game on, start up, put the monster to modify in the first position, turn the GS switch on, turn the GS switch off, check values, and save if valid. Remember this doesn't change stat EXP, EXP, or level.
So if your attack as 8, your defense was 10, your speed was 15 and your special was 11, you would (on Red/Blue):
Your hitpoint deter was (and is, in this case) 3. If you have values that you enter and your hitpoints don't come out the same, it is because you had one of them wrong by one point.
Setting W,X,Y,Z = F results in a maxxed monster.
I hope this helps.