Step one. Count what you actually have. Contacts, companies, open deals, closed deals, active pipelines, live automations, and every separate system holding customer data. That inventory is what a real quote is built from.
Step two. Decide what is worth moving. Five years of dead contacts is not an asset. Pick a cutoff date and stick to it.
Step three. Export a sample of a hundred records and look at it in a spreadsheet. You will find formatting problems, duplicates, and typos you did not know about. Better now than after they are live.
Step four. Write down your pipeline stages and who owns each one before you build anything in the new platform. Imported deals need somewhere to land.
Step five. Migrate ten records first. Check ownership, values, and stage. Only run the full load once the sample is right.