It's like the Here's Negan story, but you don't have to buy a separate magazine.

Source: Walking Dead Wikia

Here's Negan was a Negan story that ran in the Image+ magazine as a way to get people to buy the magazine. It must've worked because they're still publishing it. Negan is now ready to break out from his magazine confines to his own issue of The Walking Dead. December's #174 is a one-off story titled A Solitary Life, which focuses on the life of Negan outside of Alexandria.

And if you can't get your Negan fix before December, Here's Negan is out on October 4 in hardcover. At $20 for 72 pages, you were probably better off buying the magazine when it came out.