1. Download Kotlin.tmLanguage from here
  2. Open Sublime Merge, click Preferences -> Browse Packages
  3. Create a folder called Kotlin and place Kotlin.tmLanguage within the folder
  4. Restart Sublime Merge and enjoy! ๐ŸŽ‰

Alternatively, if you want to use terminal and you happen to be running macOS, you can run:

cd /Users/$USER/Library/Application\ Support/Sublime\ Merge/Packages/ \
  && mkdir Kotlin \
  && cd Kotlin \
  && wget https://github.com/vkostyukov/kotlin-sublime-package/raw/ce599a846ac9f86d4671a024d21220ca08e50f66/Kotlin.tmLanguage