How do you level your turntable?

Piedpiper's right.
1. Level the TT stand
2. Level the platter top surface
3. Adjust the tonearm mounting so that its bearings are level. This is easier on some arms than others of course.

Levelling the headshell is a good place to start, but whether it should remain level varies with the cartridge. Nearly all cartridges benefit from fine tuning of both azimuth and VTA, which could result in a non-level headshell, depending on the rig.

As others have posted, I also like to use a 9" torpedo level. I like to think I'm getting a better idea of the general level-ness of the platter. My Galibier doesn't have this issue but I've had tables in the past where the platters are not flat. Some either had a convex or concave surface. This can be dealt with but it is much easier to do so when you know this condition exists before trying to level.

Many levels can also be checked for accuracy by flipping them over as opposed to turning 180. Either way gives the same indication of how accurate the level is.

I used a small, straight level, not a bubble level. You need to measure in all directions - back-front, side-side, and both diagonals.

First on the table, then on the plinth, then on the platter (just spin it slightly to get all angles).

Enjoy,
Bob

A nice feature of the servo'd arm of my Sony PS X800 is that it does not need to be leveled. If you could find a way to keep the platter and the LP in place, it would work upside down!!