For the horizontal block, the process is reversed. Instead of inserting the hook from front to back, you insert it from the back of the work to the front.
Wind yarn around the hook(YO) and insert hook between 4th and 5th dc of preceeding row, from back to front. Take hook over the 5th dc, then insert it between 5th and 6th dc from front to back.
Wind yarn around hook(YO), draw it through, there are now three loops on hook, and complete the dc in the usual way. This is called BPdc.
Work around each of next three dc in the same way, alternating the groups of horizontal and vertical stitches.