After round 6, you should have a cup shape. Remember it should be inside out relative to most amigurumi, with the more ridged side in.
Pull both ends out about 1.5" and tie a knot, leaving a 5" circumference ring that won't stretch out. Here I took a picture of the final version, using the same yarn as the rest of the body.
The ring should lay smoothly along the inner loops, so you can work round 7 normally, inserting the hook under both loops and the retaining ring.