By knitting only part of the sts for several rows, it's going to curl.
So knit 8 sts of the first row, then turn and knit the same 8 sts. Next row k6, turn and knit just the 6 sts, same for the following rows where you knit on only 4 sts. Then the row after that you knit all 20 sts. Once you do this a couple of times you'll see how it works out.