There are two development models you can follow with Salesforce DX.
First is the package model where you develop against a scratch org and prepare all the components that are needed to deploy, similar to change sets but smarter as it handles the dependencies for you. We will talk about this more in the future.
The other method is what you would be most familiar with if you have been developing with Salesforce for sometime now(Force.com IDE/Mavensmate/IntelliJ), you develop changes against a sandbox and move the metadata to deploy to the target org till you deploy to production.
With Salesforce DX you can still continue to develop against a sandbox. You do not need to enable Dev Hub for developing against a sandbox or DE org.
Requirements ofcourse if you should have VSCode installed, the VSCode Salesfor ce Extension Pack and the Salesforce CLI.
Boot up VSCode and Open the Command Prompt and type SFDX : Create Project with Manifest
The scaffolding created will contain a manifest folder with a package.xml
The default package.xml adds the base ApexClass, ApexComponent, ApexPage, ApexTestSuite, ApexTrigger,AuraDefinitionBundle and StaticResource.
Next is to Authorise the Sandbox you want to work on, Go to the command pallete. SFDX: Authorize and Org.
Next right click on the package.xml and choose Retrieve Source in Manifest in Org.
Once done you can start modifying your code. Right click on a file to deploy it to the Source Org or enable the auto deploy on Save settings.
That’s it, you should now be able to work with your existing sandbox.
Check out my video and subscribe if you want more tips and suggestions.