Run the Rails server (bin/rails s) and the Webpack Dev Server (bin/webpack-dev-server) via your preferred method. Two terminal tabs will work or create a Procfile and run via overmind or foreman.

There are some good Procfile-based process management tools, including foreman by David Dollar, which started it all. The problem with most of those tools is that processes you want to manage start to think they are logging their output into a file, and that can lead to all sorts of problems: severe lagging, losing or breaking colored output. Tools can also add vanity information (unneeded timestamps in logs). Overmind was created to fix those problems once and for all.