Which Salesforce object maps to what in HubSpot?
Accounts become companies; contacts and leads, contacts; opportunities, deals; cases, tickets; and tasks and events, activities. Each opportunity record type creates a deal pipeline in HubSpot, so decide beforehand whether you keep that structure or redesign it.
Not everything has a match. Products and price books do not travel with the opportunity sync; Salesforce custom objects need custom objects in HubSpot, which are Enterprise; and the two systems' campaigns do not correspond. Decide what gets migrated, what gets archived and what gets rebuilt.
Which tool to use: the integration, Smart Transfer or imports?
The native Salesforce integration requires HubSpot Professional or Enterprise and a Salesforce admin user with API access. HubSpot recommends it to import records: it brings leads, contacts, accounts, opportunities, tasks, cases and events, with up to 500 field mappings per object.
Smart Transfer, available to Super Admins on all plans, transfers one way into HubSpot and allows one-time loads of what the sync does not cover, such as attachments. File imports handle the rest: up to 1,048,576 rows per file on paid plans.
In which order should you migrate so associations do not break?
Users first: the owner is assigned only if the HubSpot user matches an active Salesforce user's name and email. Then companies, because deals need company sync turned on and accounts already loaded. Then contacts, deals, tickets and, last, activities and attachments.
Keep the Salesforce ID on every record: the integration maintains properties such as the Salesforce lead and contact IDs, and for the rest a unique property of your own is worth having. That is what lets you reconcile, deduplicate and reload without creating duplicates.
How do you validate that nothing was lost?
With counts and samples per stage: how many accounts, contacts and opportunities there were in Salesforce, how many reached HubSpot and why they differ. Review ten random records of each object, with their associations, amounts, stages and owners, together with the team that uses them.
Also look for what gets lost silently: activities that did not arrive because turning on the sync does not bring history, Salesforce records sharing an email that HubSpot merges into one contact, and the source of leads, since migrated contacts show up as offline sources. In Chile and Latin America, keep that source in a property of your own.