I am not sure of what you mean by "migrate the leads over". If you are happy with the way the fields are mapped (In case you need to review this point, read here: Best Practices for Determining which Fields to Sync with Marketo) and the data is clean enough on the salesforce side, the next step is to enable the sync.
But before you do this, you need to ask yourself if you want all the records in SFDC to be synchronized. If not, you will have to either review the rights the the Marketo User in SFDC or establish some sync fields in SFDC and ask Marketo support to sync on these fields.
If you are interested in how filtering the sync works, there quite a few articles in the community:
Finally, there is also this nice article here: Common Marketo and Salesforce Sync Pitfalls