How do you weld a model to the character?

I am trying to make a backpack but i don't know how to weld to the player

If you have a part that holds, or is in the middle of the jetpack, make that PrimaryPart. It can be Invisible, if you don't have 1 yet, weld a new part in the middle of the jetpack, make it primary and make it Invisible.

Then add Weld To That Part and The Player's Torso.

function Weld(Part1, Part2)
	local Weld = Instance.new("Weld")
	Weld.Part0 = Part1
	Weld.Part1 = Part2
	Weld.C1 = CFrame.new(0, 0, 0) --[[I don't know what does your Model look like, but try to weld this yourself.]]
	Weld.Parent = Part1
end
Weld(game.Players.Player.Character.Torso, game.Workspace.Jetpack.PrimaryPart)