The modified half double crochet stitch is identical to a regular half double crochet, the only variable is stitch placement. Yarn over and insert your hook in between right and left legs of the stitch (rather than through the front and back loops), yarn over and pull through.
The modified half double crochet stitch is shown in the photo above, working in the round, with no turns between rounds. In the photo below, the modified half double crochet is worked with turns between rounds or rows. Either way, the stitch placement is always in between the left and right legs of the stitch.