how to accurately position armor on player?

i made a piece of armour, and testing out the left shoulder it doesnt position like how i want it to how its supposed to look; this

how it actually looks; this

what do i do?

edit: this is the script im using;

repeat wait() until game.Players.LocalPlayer and game.Players.LocalPlayer.Character


local plyr = game.Players.LocalPlayer.Character
local upperLarm = plyr:WaitForChild("LeftUpperArm")
local cloneuperLarm = game.ReplicatedStorage.ArmourLeftArm:Clone()

cloneuperLarm.Parent = plyr
cloneuperLarm.PrimaryPart.CFrame = upperLarm.CFrame

local weld = Instance.new("Weld")
weld.Part0 = upperLarm
weld.Part1 = cloneuperLarm.PrimaryPart
weld.C0 = upperLarm.CFrame:inverse()
weld.C1 = cloneuperLarm.PrimaryPart.CFrame:inverse()
weld.Parent= plyr

Use CFrames and Welding.

CFrame will position parts relative to whatever you define, Welds will keep those parts from falling off the character.