Short answer
HubSpot's KB: after connecting Salesforce, sync custom objects to keep data consistent. On Field mappings, map a Salesforce field to the HubSpot required field; HubSpot custom objects need at least one. Association fields shown reflect Salesforce's setup. Removing a record from a Salesforce Campaign doesn't trigger a sync.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
The KB notes a RecordType deletion caveat; read it before syncing RecordType objects.