Apply to become a nominator. In most cases, to my unofficial understanding, you’ll need to run a publication and have a steady reputation for having done good work for a decently long period of time.
I have been running a pub for several years and have close to 700 published stories on this platform, so some might say I’ve put my time in.
Also, just because something is nominated does not mean it will be chosen! Far from it! Nommers can nominate their own work twice a month. In my case, these are usually declined. Getting nominated is not the hard part. Getting the Boost is. I get nominated by other nommers all the time but am frequently declined. It’s becoming quite difficult to get a Boost. Just writing a quality piece that hits all the requirements is not enough. The reality is that Medium has a lot of very good writers all competing for attention. That’s actually a good thing.