Startup Guide

Install rbenv (or rvm but we're assuming rbenv for the sake of the guide)
Run ruby-build (bundled with rbenv)
Initialize rbenv: rbenv init then follow the directions it outputs. It will look something like:

    # Load rbenv automatically by appending
    # the following to ~/.zshrc:

    eval "$(rbenv init -)"

Install ruby 2.5.7: rbenv install 2.5.7. This could take awhile.
Update rbenv's shims: rbenv rehash
Install rails: gem install rails
Install the bundler: gem install bundler
You may need to install Xcode command line tools (xcode-select --install)
Install mongodb: brew install mongodb. You can choose to run mongodb as a background service with brew services start mongodb
Make the database directory: sudo mkdir -p /data/db && sudo chown -R `id -un` /data/db
Install yarn: npm install -g yarn (this assumes you have node and npm installed)
Install the node packages: yarn
Install the ruby packages: bundle install
Start the mongodb database: mongod. If you ran mongodb through brew services, you can skip this step.
In a separate shell instance, create and setup the database: rake db:create && rake db:setup
Start the rails server: rails s, or run all the dev processes in one instance with foreman start -p 3000.
If you don't like foreman's output (for example, it doesn't play nice with pry...) consider running ruby ./bin/webpack-dev-server and bundle exec guard in separate instances.

Workflow

Make sure your packages are updated: yarn and bundle install
Start the mongodb database: mongod. you can skip this if you have mongodb running as a background service (brew services start mongodb)
In a new instance, setup database: rake db:setup && rake db:create_indexes
Start the rails server: rails s, or start rails, guard, and webpack-dev-server in one instance: foreman start -p 3000
If you don't like foreman's output (for example, it doesn't play nice with pry...) consider running ruby ./bin/webpack-dev-server and bundle exec guard in separate instances