Buying a rental property with a tenant in place but no lease.

[Palmer Reap]

Hi all! My fiancé and I are closing on our first real estate deal and are buying a duplex so that we can house hack (thanks, Scott Trench!). One of the units already has a tenant in place for 8 years now who pays timely and is extremely cooperative. He is basically a tenant-at-will and does month-to-month though again there is no lease in place. My question is how should I go about putting a lease in place and are there any suggestions for making it as smooth a process as possible? He appears to be a good tenant so I would like to keep him if at all possible! Any advice would be greatly appreciated.