1. Your circle is becoming like a bowl - add more single crochet stitches into the round (do more of twin single crochet at one stitch)
2. Your circle is becoming wavy/not flat - reduce single crochet stitches on the round (do less of twin single crochet at one stitch)
3. Your circle is kind of flat, but not exactly even - try ironing it ;)
I hope that helps!