The first emphasis in Make Fewer Things Matter is “make.” Things don’t stop mattering on their own. You don’t just ignore them. You do something to make them not matter.
The next emphasis is “fewer.” Some things will still matter, but you reduce the number of them. Make a big list of things you think are important. Look at each item and look for ways to make it not matter.
After you go through everything and you try to make them not matter, you’re left with a few things that truly matter.