Round Three of Square
If you are going to keep going around with the same color from round one, then slip stitch up to the top right corner. That’s the closest corner to where you joined. When you begin Round 3 and all future rounds after this, you will begin in that same top right 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. (The corner shown in the picture below.)
If you are going to use the same color in the next round, slip stitch to the nearest corner, which will be your top right corner. If this is as large as you want the square or if you are going to switch colors, then fasten off from this color and join your new color with a slip stitch in that top right corner.