I don't have too much help... I use a drill press and 'flap wheel', and I do a lot more regraining lock down bars than ball guides.

I set up a piece of wood that I clamp to the drill press table to make a flat table to keep things absolutely straight as I slide the piece that is attached to a wood block along the table surface into the spinning flap wheel. By sliding on a flat surface I keep the lines straight.

The key is to never slip, never end your stroke too early. Repetition and being careful.

You can get good results, but it's always nerve wracking when you have to keep things absolutely straight.

Start with course sanding material, then progressively finer, then eventually finish off on a polishing wheel.

Sometimes I do abrasive ball, flapwheel, or just hand sanding--all that generally depends on the shape of the part, though. When you start getting to a finer grit with hand sanding, I generally do wet sanding. That followed by polishing can lead to something close to a mirror finish.

As for the motion--it's just back and forth along the long face of the metal part. Don't slip and don't go off at an odd angle, and it usually turns out fine.