The simplest way to install Jekyll is using Homebrew if the operating system is MacOS. Assuming that the Homebrew is already installed, we need make sure that it is up-to-date.

Also, Jekyll is built on top of Ruby. We might not want to mess around with the Ruby installation that comes with the MacOS. So it is preferably to install rbenv first.

brew update
brew install rbenv
ex -sc '1i|export PATH=$HOME/.rbenv/bin:$HOME/.rbenv/shims:/usr/local/bin:$PATH' -cx ~/.bash_profile
rbenv local 2.5.3
gem install jekyll
gem install bundler