How do i weld models to a player?

To weld two parts together we first have to create the weld, then change its Part0 & Part1 properties, change its C0 property, a CFrame that controls how Part1 is moved and orientated from Part0, and then parent it.

local char=--character
local P0=char.UppTorso --what the model will stick to
local P1=--a part in the model

--Rotation is in radians, you can use math.rad(degree) to to use degrees if you want
--e.g    CFrame.Angles(math.rad(90),0,0)

--     Moves 0,0,0                  Rotates 0,0,0
local Dif=CFrame.new(0,0,0)*CFrame.Angles(0,0,0)

local w=Instance.new("Weld")
w.Part0=P0
w.Part1=P1
w.C0=Dif
w.Parent=Part0