I can't opine on MN law, so this is a general description that may omit some features of specific states' laws.
The short answer is to file an LLC with the secretary of state.
As a single-member LLC the IRS considered it a disregarded entity, and you can call it that on the SS-4 form when you get its tax ID just so there's no confusion.
After that, retitle any business assets in the LLC's name. Change insurances over. Change licenses, utilities, bank accounts, etc. over.
Mid-year is fine.