Okay, I found a post with screenshot, and the group rules don't mention not reposting, so here it is (if I can get the image to insert!)
All right, I can't figure it out. I got c*cky when I saw the 'insert image' icon, but it wants a link, and I don't think a link to the image will work unless you're a member of the group. Sorry.
I did however, learn that you access the new DIY option by clicking on the three dots beside the Promote and Advertise button on your book in the KDP Bookshelf. From the screen shot, it looks like the new option is called 'Manage Series'.
It's quite possible you already have access to it, even though I don't yet. It would appear they are doling it out gradually.