How do we make Inverse Kinematics?

I never asked for the damn code mods so don't even try closing this. Just need some help with IK kinematics (aka dynamic movement) the problem is I don't know how the hell to make it and I get errors in the process of the code.

Code:

wait(1)
User = game.Players.LocalPlayer
Humanoid = User.Character.Humanoid
Torso = User.Character.LowerTorso
L_Hip = Torso["Left Hip"]
R_Hip = Torso["Right Hip"]

local step = game:GetService('RunService').RenderStepped;

function ArcTan(x, y)
    r = math.atan(y / x)
if x < 0 then
    r = r + math.pi
end
return r
end

vel = Vector3.new()
while true do
    step:wait()
    local velVector = Vector3.new(Torso.Velocity.X,0,Torso.Velocity.Z);
    if velVector.magnitude > 0 then
        m = (vel - Torso.Velocity).magnitude / Humanoid.WalkSpeed
        vel = (Torso.Velocity * m + vel * 2) / (m + 2)
        v = Torso.CFrame:pointToObjectSpace(vel + Torso.Position)
        L_a = ArcTan(v.x, -v.z)
        R_a = 3.1416 - ArcTan(v.x, v.z)
        L_Hip.C1 = CFrame.new(0, 1, 0)*CFrame.Angles(0, L_a+math.pi, 0)
        R_Hip.C1 = CFrame.new(0, 1, 0)*CFrame.Angles(0, R_a+math.pi, 0)
        L_Hip.C0 = CFrame.new(-0.5, -1, 0)*CFrame.Angles(0, L_a-math.pi, 0)
        R_Hip.C0 = CFrame.new(0.5, -1, 0)*CFrame.Angles(0, R_a-math.pi, 0)
        L_Hip.MaxVelocity = Torso.Velocity.magnitude / 100
        R_Hip.MaxVelocity = Torso.Velocity.magnitude / 100
    else
        L_Hip.C1 = CFrame.new(0, 1, 0)*CFrame.Angles(0, -1.57, 0)
        R_Hip.C1 = CFrame.new(0, 1, 0)*CFrame.Angles(0, 1.57, 0)
        L_Hip.C0 = CFrame.new(-0.5, -1, 0)*CFrame.Angles(0, -1.57, 0)
        R_Hip.C0 = CFrame.new(0.5, -1, 0)*CFrame.Angles(0, 1.57, 0)
    end
end