You would do short rows. On the row before the BO, stop 4 sts from the end, turn and knit the RS row. The next row, stop 8 sts from the end (4 sts from where you quit before), turn and do the RS row. Since you only have 12 sts, stop right there and all the sts that should be bound off are now on your needle and ready to finish.
sue- knitting heretic