To practise half stitches, continue with your practice piece or start a new one.
If you start a new one, begin with a couple of rows of half double trebles. As with the extended trebles on your practice piece, if you made one, it will give the fabric a sturdy foundation and a base row of ‘normal’ stitches to compare your extended stitches with. Work 13 hdtr (ch15, hdtr in fourth ch from hook and in each ch to end, turn, ch3, hdtr in each tr across, turn).