advice for a 90 degree vertical slide? (if I understand your question correctly)
First take the handles and turn them (thumb direction) to face the same side (L or R) as the leading edge will be facing.
Now stack them, one above the other. From this position imagine what the flight path should look like, remembering that a slide is created by the "difference between the handles" not wrist rotation. One must be pushed forward (the bottom handle) and the other must be drawn backwards towards your shoulder. Maximum forward lift is effected from this fully extended position and relative positioning relationship of the handles is still in-line. (it's called the archer move, you can see why!)
To get the kite to hover (or descend) you will change the positioning of the two handles from the extreme example above used for maximum lift speed (up), but remember the handles are stacked one above the other. (Okay it's actually an arc, but it's in alignment)
Go ahead, rotate your torso and reposition your feet, so all of this is comfortable. One of the handles is creating the stability (L to R) and the other is controlling the vertical height adjustments. Which one? You'll figure it out in short order, it's personal.
Okay now, go affix your flying lines again and practice. The easiest starting point is a tip stab,... the secret to returning to the same spot is to watch a mark on the ground instead of watching the kite directly. Keep your intense focus on that spot, flying only in your peripheral vision. Practice the vertical slide on both sides too and your lines had better be even!
Make sure you have enough "down" in your leader tuning, so the kite is hovering at neutral and you are in a comfortable & relaxed position physically.
It will get better & better with practice,... before you know it you'll be doing sliding vertical threads this way with dozen of other pilots.