How do I fix my first person camera script?

I'm making a custom first person camera script. As apposed to using roblox's so I can add smooth head bobbing, crouching etc. However, I'm bad at 3-D math so I'm stuck on how to make the camera turn with the mouse. What happens is the camera starts twisting and going out of control.

while not game.Players.LocalPlayer.Character do wait() end

local cam = workspace.CurrentCamera
cam.CameraType = "Scriptable"

local plrChar = game.Players.LocalPlayer.Character

local plrHead = plrChar:waitForChild("Head")
plrHead.LocalTransparencyModifier = 1

local CAMERA_MOVE_FACTOR = 3

local xAngle = 0
local yAngle = 0


inputService.InputChanged:connect(function(input)
    if input.UserInputType == Enum.UserInputType.MouseMovement then
        if input.Delta.x < 0 then
            xAngle = xAngle - CAMERA_MOVE_FACTOR
        elseif input.Delta.x > 0 then       
            xAngle = xAngle + CAMERA_MOVE_FACTOR
        end

        if input.Delta.y < 0 then
            yAngle = yAngle - CAMERA_MOVE_FACTOR
        elseif input.Delta.y > 0 then       
            yAngle = yAngle + CAMERA_MOVE_FACTOR        
        end
    end
end)

game:GetService("RunService").RenderStepped:connect(function()
    print(CFrame.Angles(math.rad(-yAngle),math.rad(-xAngle),0))
    cam.CFrame = CFrame.new(plrHead.CFrame.p) * CFrame.Angles(math.rad(-yAngle),math.rad(-xAngle),0)
end)