How shall I prepare for my M50's?

Just play them at a moderate to moderately loud listening level with music you normally enjoy.  The easiest way for me is burn them in with HD radio, but FM will work too.

What I would recommend is play it at 50% of it's max volume with any songs you wanted. Please be noted that do not listen it when at that volume! For each day, burn them in for at most 2-4hours. Avoid anytime longer than it a day. Burn them in with the same process & instruction for a week or two should be fine. Sound will stop changes at around 40-50hours to me.

Just my 2cent

While I'm not an advocate for or against burn in, I can tell you that my set of M50's did not change much if any after the first 2 hours of use.  They were a tad sloppy out of the box but tightened up a noticeable amount after 90 mins of use.  Enjoy!
 

The best way to burn in is to play a constant stream of pink noise.

I like to perform a naked dance under the moonlight while wearing my new headphones, then I sacrifice a pair of pure silver cables to the audio gods. Helps to remove sibilence.

Must be tough to do this time of year.

That's why I buy headphones in summer.

Use gold cables.

I never fully understood this need for fat burn-in. I do think that burn-in is good for headphones to get better with time (my AKG701 got remarkably better after a few hours at moderate levels). But the good thing about it is feeling the improvement, isn't it? What's good about blindly plugging your headphones in some source for pink noise, and just wait 100 hours?

 

I on the contrary think that "active burn-in" is much more enjoyable and teaches you a lot about the difference in sound and makes you understand and appreciate the details in sound. In other words, just plug you headphones in and enjoy your music. You'll burn-in with your headphones.