How can I measure my PD. And how is it done?
Center to center or middle of face to center.
And just how important is it?

Middle to centre – could be different for each eye. Done with a ruler and the trained eye of someone who knows what they are doing as far as I can tell. Important as it centres each lens to the eye (as the frame is effectively symmetric) – no idea what difference it makes, but my PD is different by a good few mm eye-to-eye, as it were.

Order new glasses online are we?

Tape measure just above/below your pupils. Get someone to take a digital photo. Zoom right in and read off the measurement from the centre of one pupil to the centre of the other.

free pd ruler

From what I have been told its not a great idea, only need to be slightly out to mess up the glasses fit and even more risky measuring to buy contact lenses online

You don’t need to measure PD for contact lenses.

I did my own measurement that simply measured the outside of the left to inside of right (or something like that) and they were fine.

http://www.specsonthenet.com/measuring-PD.jsp