Kara cancelling the super off a crossover is the same way you would kara cancel c.jab x2 > super
To kara cancel c.jab x2 into a super you charge D or DB and press c.jab > c.jab > QCUB+jab~short
A shortcut method of doing guile’s super is to charge D or DB for it, then do a reverse tiger knee motion+K. For you Marvel players, this is just like doing AHVB.
The key to getting this to work off of a crossover is to charge for the super FK from the DOWN position, straight down. It’s often not known that guile can charge for his super this way. So, just jump and your opponent and immediately hold down, do the crossover short, then c.jab > c.jab > QCUB+jab~short kick.
There is an alternate method I use to do guile’s super FK that can make it easier to perform in certain situations. I charge straight down, the quickly press DF, UF+K. I think of doing the super the same way I would do a dash in 3S or Marvel. I kind of charge down, then double tap F, F +K(only using DF, UF). Sorry if this doesn’t make sense. I still don’t know the best way to explain it.