Gridshot is done standing still, in valorant you're gonna have to be constantly moving and able to stop and shoot at a small target in quick succession
Improving your aim definitely helps, but I also recommend doing a bunch of deathmatches until you internalize where head level is for most angles that you'll peek, that way you only have to move your mouse horizontally when you want to kill someone and not have to memorize a bunch of ways to flick someone vertically at an angle.
my #1 tip is to improve your horizontal tracking with a very small target (I suggest learning to move your mouse with your arm a bit more; the best way to do this I've found is to do a bunch of gridshot with just your arm making sure your wrist is stiff, and make sure that you only move in straight lines and don't wiggle around too much)
make sure your aim is as least shaky as possible, be able to move front left to right in a horizontal 180 degree line as best as possible and once you get it practice following faster and faster targets. From there your crosshair placement and horizontal flicks are theoretically all you should need to get good. You'll need to learn to strafe and combine your aim with movement in the actual game as well, although some tracking scenarios allow you to move so I recommend finding one like that.
optionally you can try adding verticality to your skillset for situations like jett updrafts or simply getting caught off guard by someone on an off-angle on top of a box or something, but that bears less fruit imo
P.S. Don't expect to see results after just one session, give it like a week and you'll notice visible improvement