Let's say you have some footage that's supposed to be a lock-off but somewhere during the filming the camera gets nudged, or bumped or slowly drifts due to a loose lockdown or a sleepy operator.
If it's a short clip and you can't really use smoothcam or some other image stabilization software due to them digitally zooming in I'll do this:
1. Download and install the plugin "DH Grid" from here.
2. Layer that atop the less-than-smooth footage.
3. Resize and reposition the DH Grid overlay so it's grid aligns with a portion of the image that you want to stay where it is.
4. Set a Center keyframe in the Motion tab of the clip.
5. Now using the JKL keys, advance frame-by-frame (or small groups of frames) and Nudge the footage up, down, left and right as needed using the grid to keep it in place.
One great way to do this is to use the little known Nudge key commands in FCP: Option + Arrow Keys. It'll help if you have the Canvas set to Image + Wireframe and you must have the clip you're nudging highlighted in the Timeline.
If you want to nudge a really really tiny amount use Command + Option + Arrow Keys. Depending on how zoomed in you are -or aren't- in the Canvas you may need to zoom in to see these tiny adjustments.
6. Each time you nudge the clip on a particular frame you'll automatically set a keyframe.
7. Render and see how badly you messed up :) Don't worry, it's hard to get it right the first time. And the human eye/mind is so used to seeing movement that is natural, that unnatural movement will seem quite obvious.
Scoot back to the problem frames and adjust them as needed, render, wash, rinse and repeat.
Be aware though that if you've fixed a particularly nasty bobble, you're first inclination after making all these keyframes is to zoom the clip or reposition it (since the bottom or sides or top may be cut off).
If you move the clip now you'll set another Center keyframe and mess things up.
Instead, use this little-known key command: Hold Command + Shift and move the clip in the Canvas. Command + Shift will move the clip AND THE KEYFRAMES en masse without setting any new keyframes. Nifty, eh?