Kotlin can be installed using Choco on Windows, Homebrew on Mac and wget on Unix. I chose to download it and so, after extracting the files, I had to add the kotlin/bin folder to the PATH on my machine. Java is, of course, prerequisite.
From the command line, entering kotlinc starts the REPL.
C:\>kotlinc
Welcome to Kotlin version 1.3.31 (JRE 1.8.0_191-b12)
Type :help for help, :quit for quit
>>> fun greet(name: String, greeting: String = "Howdy"): String = "\n$greeting $name"
fun greet(name: String, greeting: String = "Howdy")>>>
>>> println(greet("Greg"))
println(greet("Greg"))
Howdy Greg
>>> println(greet("Sue", "Bonjour"))
println(greet("Sue", "Bonjour"))
Bonjour Sue
>>> :quit
C:\>