Help with welding to Specific player?

im creating a simple part (union) and trying to weld it to the players right arm. i am doing this with a script and i am not quite sure how to do it.

for future reference, how could I weld multiple parts together onto somebody?

if I can explain this more clearly, Roblox murder mystery 2 and similar games all have a knife welded (i think) to the back of the player.

if I manage to get the welding to work, how can I get the object to be ON the player, and how can I move the object for say toward the upper part of the right arm? i have not found a tutorial on youtube that can help weld things onto the actual character.

this is my code so far:

game.Players.PlayerAdded:Connect(function(plr)
	if plr.Name == "Nimbzee" then
		local light = game.Workspace.Union
		local weld = Instance.new("Weld")
		wait(2)
		plr.Character:WaitForChild("Humanoid")
		plr.Character:WaitForChild("Right Arm")
		weld.Part0 =  game.Workspace.Union
	end
end)

please note i dont really know what im doing as i have never made a weld script before. any help would be much apreciated

i don't know if this helps but I'm using r6 characters

https://www.youtube.com/watch?v=yBNHbNFK5nQ

Same concept, instead do it with a knife, weld parts together using WeldConstraint if your accessory uses more than 2 parts.