A good question. I'm not sure but i presume simply sitting, relaxing and focusing on nothing.
So not the Christian style praying.
Real meditation is not blanking the mind (which can never be done, anyway). Traditional meditation involves meditating on God. That's what the ancient Vedas teach. There is no possiblity of ever successfully meditating on "nothing".
And just picking any old thing to concentrate on? Everyone does that all the time anyway. Can be called "obsessing"!
:-)