Oh yeah, that sort of rig is a complete nightmare to use handheld, especially with film. I'd start with the lens on its own at first, without any tubes or modifiers.
With tubes:
- Set your lens focus to infinity; the focus adjustment ring will only provide very minor adjustments at magnifications greater than 1:1 and may actually degrade lens performance at other settings.
- Adjust your plane of focus (which will be almost nonexistent) by physically moving closer to or further from the subject... on the order of millimeters.
- Stop down as much as possible. Less than zero DoF or hitting diffraction limits are your only options because ...
- ... you'll lose several stops of light due to the tubes and be unable to achieve hand-holdable shutter speeds.
This is a digital image, but illustrates a similar configuration. Nikkor 105mm + 1.4x TC + 68mm of extension tubes.