Replying to Measure your own feet

So here's my thinking...

Every time somebody posts in gear talk about buying boots online, the first response is go see a boot fitter.

Now I entirely get this, you want the boots to fit your feet exactly and so you want to be able to check that your measurements match the boots.

So here's my question...what measurements do you need to figure. Don't get me wrong, good boot fitters are great. They can identify why your boots are hurting and they can make the fit exact etc. But given you can find online discounts which are huge, what measurements would I need to take in order to have a good idea what boots online would fit (or at least be a close enough fit that a liner would make them perfect)?