When you say doing it from a hop, I'm guessing that you're talking about both a tigerknee V-slaher (hop into instant V-slasher) and doing the V-slasher a little later during a hop.
For instant V-slasher, I have two main ways of doing it. If it's a predictive/psychic/"man alive I hope this hits" situation, I input qcf,u/f,f,d/f,b+P. If I'm doing it to try blowing up a sloppy frametrap or mistimed meaty, I input qcf,hcb,u/b+P. A couple of other methods for instant V-slasher are qcf,qcf,b,u/b+P, qcf,u/f,qcf,b+P, qcf,u/f,qcb+P, or qcf,qcb,u/b+P.
For V-slasher later in a hop/jump (like trying to cancel it off of an air normal), I just go for qcf,d/f,b+P, since I can do that the fastest.
As for how to physically do the inputs, that's something you have to figure out for yourself by just trying until you figure it out. I would suggest trying not to ride the gate, though, especially with an octagonal gate (because it has a larger opening than a square gate), since that will slow down your inputs.
