Currently, using a child-theme to extend a Lumberjack theme is unsupported.
Using the Installer (Recommended)
This is the recommended way to create a new site and assumes you want to build on top of Bedrock. If you're new to using WordPress with Composer this is the simplest (and quickest) way to get up and running as the installer will do all the heavy lifting for you.
Download the Lumberjack Bedrock Installer with Composer (you only need to do this the first time):
composer global require rareloop/lumberjack-bedrock-installer
Make sure that Composer's global vendor bin directory is in your $PATH so that the installer can be used.
Create a new site
Once the installer is available on your computer you can create a new Lumberjack/Bedrock site using:
lumberjack-bedrock new my-site
This will create a new folder my-site in the current working directory, install the latest version of Bedrock, add the lumberjack-core dependency and download the most recent Lumberjack starter theme.
To complete the install do the following:
Edit the .env file and update environment variables:
DB_NAME - Database name
DB_USER - Database user
DB_PASSWORD - Database password
DB_HOST - Database host
WP_ENV - Set to environment (development, staging, production)
Lumberjack can be added to an existing theme as long as Composer is being used as part of your setup. You're able to use as little or as much as you need and the framework will play nicely alongside more traditional WordPress code.