There are multiple ways to onboard a merchant into commerce Facebook family of apps:
Commerce manager redirect + OBO
Manual onboarding is very similar to a direct API integration. It requires onboarding via Commerce Manager and then, in addition, a permission setup for your application to manage assets (catalog, page, pixel, etc.). This method is lengthy and requires work with a merchant. It is a good way to get familiar with our platform, but will limit your scale opportunities.
This is an automated way to provide same onboarding.
You will need to implement UI to provide Facebook Login and redirect partners into the Commerce Manager.
When the merchant finishes with the Commerce Manager setup they will be redirected back to your platform to continue work.
You would need to execute OBO to have access to the newly created commerce account.