I've searched for the tutorial on Youtube and I couldn't find one that is working. All I saw was a Script on making the Part rotate. I've been having this problem for a long time now and couldn't make what I wanted to make on Roblox Studios.
You have to set a part in that model as the primary part with:
script.Parent.PrimaryPart = script.Parent.Your_Part
then you can set that primary parts cframe with:
script.Parent:SetPrimaryPart(script.Parent.PrimaryPart.CFrame * CFrame.fromEulerAnglesXYZ(x,y,z)
well its a pretty easy thing to do, if u only want one part to rotate then u make that part and insert a script with it, inside that script u simply just type
script.Parent.Orientation = script.Parent.Orientation + Vector3.new(numbers in here)
and put that into a loop for it to rotate all the time