I can't think of a buttonhole on a pair of trousers that you wouldn't prefer to be a keyhole buttonhole, as opposed to a standard shirt-type buttonhole. A keyhole buttonhole won't get distorted from the width of the wrapped thread shank you should be making behind any button on your trousers; the "key"-hole at the end accommodates the shank, or should if it's big enough Details in book, as usual.
Many recent sewing machines offer keyhole settings, but I've never seen one I really like (if you like yours, please send me a picture of one; I'll post it). The problem with the ones my machine makes is that, like most machine-made ones I've seen, the hole at the end is too small to be useful. Also, my machine makes the hole with satin stitches that don't radiate around the opening; they remain parallel to the stitches along the sides, which makes the whole thing look amateur; nice try, but no thanks! It's a nit you may reasonably find not worth picking, but improving on these lame holes is pretty easy, if you wish.
On the DVD that comes with the book, you'll find detailed directions and videos describing how to use an eyelet plate to create really nice keyholes, with stitches that wrap over and radiate around a precut hole. But for this sample, I decided to just go freehand, building a reasonable facsimile of a keyhole with 4 angled, slightly curved lines of narrow satin stitching. Keeping the two sides symmetrical and the whole thing accurate was managed with a few dots from a disappearing marker, one at each pivot point around the hole at the end.
Note that I didn't cut all the way to the narrow end, and that I allowed for this by making the hole a bit longer than I needed, and kept the stitching close and parallel in the circles area for a short distance, forming a sort of double bartack there. The whole thing was about twice as fast to complete as it would have been to set up my machine for a pseudo-keyhole, and took about a fifth the time an eyelet keyhole would have taken.
When attaching the band-with-tab to the pant front, I clipped the band fabric right at the point where I wanted the tab to begin, having stopped the hand felling a little ways short of this.
After joining the band to the front, I pressed open the trimmed garment/band allowances, blending the garment allowance smoothly open about a half-inch [1.2cm] from the pocket mouth. Note that the petersham there also blends smoothly, from folded to match the tab, to not-folded above the pocket. This blending is shaped to conceal the blending of the pressed-open seam allowances underneath.