Actually you have to cast on sts this way at the beg of a row, not at the end. So turn your work, knit into the first st but leave it on the needle and put the new st on the L needle also. Do this to make 10 new sts, then work the new sts and the rest of the row. Turn and cast on 10 sts at the beg of this row too. You want 20 new sts (97 + 20 = 117) so there's 10 on each side.