How do I get this to move in motion rather than jumping to a vector3 number?

for i = 1, 5, .5 do 
wait(1)	
		script.Parent.Rotation = script.Parent.Rotation + Vector3.new(0,-10,0)
wait(3)
script.Parent.Rotation = script.Parent.Rotation + Vector3.new(0,-10,0)
	
end

The Title may not seem like it makes sense, but what I'm trying to do is. I was trying to make this "Strand" of grass to tilt, left to right. Or at least look like it is doing that. Everytime I do this script it goes directly to -90 on the Y-AXIS and then goes directly to -80 on the Y-Axis. And if I put this script in a While True Do will the the script continuously do the script again while keeping the same flow? Thanks for the help. If you don't understand, "Comment."

This script will do something like that, you can change these numbers to finetune the movement. Lets say you do for i = 1, 20 do at line 2 then you change the -1 at line 4 to -0.5

function tilt() -- We add this to a function so it can be called more then once
	for i = 1, 10 do -- We make a loop at let it run 10 times
		wait(0.1) -- Wait for a short period
		script.Parent.Rotation = script.Parent.Rotation + Vector3.new(0,-1,0) -- Rotate just a bit
	end
end

tilt()
wait(3)
tilt()