How do i temporarily weld part to a player?

Think of welding as anchoring a part, except instead of it floating there not moving, it stays attached to whatever it was told to stick to, even move around with it if you'd like

Judging by the fact you'd like to weld to a character this is what it would look like, and for the example, we'll do an angled sword attached to the back.

The first step is to index the Objects you're working with, for this transaction, we're looking for:

--// the torso, and sword
local Player = game:GetService("Players").LocalPlayer
local Character = player.Character
local Torso = Character:FindFirstChild("Torso")
local Sword = workspace:WaitForChild("Sword")

Next, we need to create the weld instance

local Weld = Instance.new("Weld")

After that, we need to start using this weld to bind both objects together. This is what the process looks like:

--// Before working with the Objects though, we need to make sure the Object that's going to attach to the other isn't anchored. Next, since we're giving it an angle, and a position on the back; CFrame, we have to set this first.

if (Sword and Character) then --// Extra precautionary measure
	Sword.Anchored = false
	Sword.CFrame = Torso.CFrame * CFrame.new(0,0,1) * CFrame.Angles(0,0,45)
end

Now we set the weld:

if (Sword and Character) then --// Extra precautionary measure
	Sword.Anchored = false
	Sword.CFrame = Torso.CFrame * CFrame.new(0,0,1) * CFrame.Angles(0,0,45)
	Weld.Part0 = Torso --// Part0 is the Object the other Object is welding to
	Weld.C0 = Torso.CFrame:inverse() --// Makes this object follow the CFrame of the Object it's going to weld with, this is very important
	Weld.Part1 = Sword --//Object thats going to attach to the other
	Weld.C1 = Sword.CFrame:inverse() --//Now that their CFrames are binded, they're seem as if it's completely following, or in other terms mimicking.
end

Attach everything

local Player = game:GetService("Players").LocalPlayer
local Character = player.Character
local Torso = Character:FindFirstChild("Torso")
local Sword = workspace:WaitForChild("Sword")
local Weld = Instance.new("Weld")
if (Sword and Character) then
	Sword.Anchored = false
	Sword.CFrame = Torso.CFrame * CFrame.new(0,0,1) * CFrame.Angles(0,0,45)
	Weld.Part0 = Torso 
	Weld.C0 = Torso.CFrame:inverse()
	Weld.Part1 = Sword
	Weld.C1 = Sword.CFrame:inverse()
end

Hope this helps