TIMING CHAIN MARKS QUESTION?

I watched several videos on how to go about it. I just want to change the gasket but I have to remove the chain to do that anyways, if I see a chain stretch I may change it. I saw the tool which is not expensive to lock the cams and crank. My question is, I have not seen one video that mentions what to do if say the timing was off by one tooth, also does a stretched chain always mean the timing is off? I have no timing codes and it runs well. If it is off and the cams are locked how do I go about turning the camshaft so the slots on the back of engine are flat.