All you have to do is weld all parts of the model to the primary part of the model.
And make sure that none of the parts in the model are anchored except for the primary part.
I would Use :GetPivot() and :PivotTo() for this, instead of the Tween Service.
An example, A model rotates 365 degrees over the X, in 3.65 seconds:
local Model : Model = nil --replace your model in here
local RotationNumber = 365
repeat
RotationNumber -= 1
Model:PivotTo(Model:GetPivot() * CFrame.Angles(math.rad(1), 0, 0))
task.wait(0.01)
until RotationNumber <= 0
I guess my question is why are you trying to Tween it?
(I am assuming your using the Tween service, though I don’t know if you are!)