Step 1: Pick up stitches
You will noticed a slight crease in your amigurumi right around the waistline from the original 30 stitches that you crocheted. That crease is where you are going to pick up stitches for the dress. Pick up 30 stitches and single crochet them in a row around your ami's waist.
Step 2: Single crochet 2 rows even. Your dress will look a little funny, kind of like a little bubble dress (see below), but don't worry.
After the 2 even rows, you are going to start increasing slightly. Just do one increase on each side of the dress, indicated by the arrows below. I like to increase on the sides so that the front of the dress is unobstructed.