M1 can make transfering another brokerage account to an M1 account easy. To initiate a transfer, please email firstname.lastname@example.org. In the email, include a copy of your most recent monthly statement from the brokerage account you'd like to transfer to M1 and any specific instructions for the transfer. After you send the email, our transfers team will handle the rest.
Attached to this article is a PDF that describes the account transfer process in detail.
If you have any specific questions, never hesitate to email the team at email@example.com.