A horizontal crocheted buttonhole is generally made in a row of sc's simply because sc's have a little height but not much. And so, the buttonhole would have a little height but not much. The bottom of the buttonhole is the top of the last row. (Skip as many sts as the buttonhole is wide.) It doesn't have to be a row of sc's as it is here. It could be a mesh or a row of dc's or .... The top of the buttonhole is made by ch sts.
Here's what the mesh from last week looks like -- unstretched and unblocked. I ended up with 15 sts across. Then, for this go-around, this is what I did next:
Turn, ch2, sc across.
Turn, ch1, sc4 (I did ch 1 instead of ch 2 to give the edge a more rounded look. I'm going to end the row with a slip st to give the other corner a rounded look as well.)
The next question is how many ch sts to make to create a buttonhole.
The general answer is "however many sts skipped". However, ch sts stretch more than sc sts. The second picture shows this.
For the third picture, I did 1 fewer ch st than I skipped in the previous row.
And here's the final picture.
The last row is:
ch 1, sc 4, ch 4, skip 5, sc 4, slip st 1.