1. Create mymodule.breakpoints.yml in your module's base directory. This file will hold the breakpoint configurations.
2. Firstly, we will add a standard mobile breakpoint that does not have a media query, following mobile first practices:

mymodule.mobile: 
  label: Mobile 
  mediaQuery: '' 
  weight: 0

3. Secondly, we will create a standard breakpoint that will run on a larger viewport:

mymodule.standard: 
  label: Standard 
  mediaQuery: 'only screen and (min-width: 60em)' 
  weight: 1 

4. Thirdly, we will create a wide breakpoint for devices that have a large viewport:

mymodule.wide: 
  label: Wide 
  mediaQuery: 'only screen and (min-width: 70em)' 
  weight: 2 

5. Your mymodule.breakpoints.yml should resemble the following:

mymodule.mobile:
  label: Mobile
  mediaQuery: ''
  weight: 0 
mymodule.standard:
  label: Standard
  mediaQuery: 'only screen and (min-width: 60em)'
  weight: 1 
mymodule.wide:
  label: Wide
  mediaQuery: 'only screen and (min-width: 70em)'
  weight: 2 

6. Go to the Configuration tab and then to Performance to rebuild Drupal's cache and make the system aware of the new breakpoints.