If you are going to change colors, or if this is as large as you want your square, then you can fasten off now.
If you are going to keep going around with this same color, then slip stitch up to the top right corner. That’s the closest corner to where you joined. When you begin your next round, you will begin in that corner. So you either slip stitch to that corner with the yarn you are using now, or you’ll simply join your new color with a slip stitch in that corner. I will explain more about that in the next part where we talk about round two, but just so you understand the concept of where this is going I thought I’d mention it now.