I don't have that pattern, and it would help if you would copy that part of the instructions for us. That said, my guess is that you skip down one row, or two, grab the front loop of the stitch in that row below, and crochet into it several times as you do for a cluster, probably using double crochet. Finally, you make the last loop on the stitch, having had as many as 6 or so loops as you make the popcorn stitch.

OK, here goes:

Popcorn Strip - Make 4

With afghan hook, ch 33 and work row 1 and first half of row 2 in plain afghan stitch. Yo and through first loop, *yo and through 2 loops twice, ch3 (for popcorn)*, repeat from *to* 6 times more (7 popcorns); yo and through 2 loops 3 times, ch 3, repeat from *to* 6 times more, ending with yo and through 2 loops 3 times. Row 3 and all odd rows: Work plain afghan st., keeping popcorns on right side. (33 loops)

Well, I have done the popcorn stitch, but never in an afghan stitch using a long afghan hook. Did you change the wording of the instructions? It does not read as precise as a pattern usually will. I am unable to help you with this wording as I don't understand it myself.

I have knit popcorn stitches and have crocheted them, but never using the afghan or Tunisian stitch.

Thanks so much for your response. What really bugged me about this is that I actually made this same afghan about 40 years ago from the same pattern! My daughter who is out of state has it so I looked at it when I visited her last week. I figured it out once I saw it, but those directions were not very explicit.

The tricky part was that 'one half' of a row in the afghan stitch is half of a two-part row which all rows are. Anyway, I'm going to finish making my afghan the way I originally wanted (with the popcorns!)