First, you need to install pry-byebug gem. Run this command:

$ gem install pry-byebug

Add this line at the top of your .rb file:

require 'pry-byebug'

Then insert this line wherever you want a breakpoint:

binding.pry

A hello.rb example:

require 'pry-byebug'

def hello_world
  puts "Hello"
  binding.pry # break point here
  puts "World"
end

When you run the hello.rb file, the program will pause at that line. You can then step through your code with the step command. Type a variable's name to learn its value. Exit the debugger with exit-program or !!!.