I've found not all sweet potatoes will grow. I look for ones that have growth already. Usually small bumps, sometimes the tips of vines started. On ones I haven't seen at least tiny bumps have rotted. I would start at least a month before it's time to plant though, some of them can take some time to get going. Once they start to grow though they really take off. Make sure to change the water every few days or else they may begin to rot. Or up them into a pot of dirt. I've done both and they both work. Though I like to start them in water then pot them up once they get going.
It might be too late to start slips for this year's harvest, Seed. But you could "practice" starting the slips so you will know what you are doing next spring. It took mine forever to start to grow this year, and I had used organic potatoes, so I know they weren't treated. I finally just planted the potato in potting soil. Once they start growing, you can cut off a section of the vine and root it in water, for another plant to put in the garden. Also, I found that you can stick a rootless cutting in the ground, keep it watered, and it will grow as well as the rooted ones.