Adding an in-seam pocket: couture method
Here's a way to add an in-seam pocket with the construction entirely on the front of the garment. This gives a reliable result generally, with the pocket sitting flat, and can be used to add a pocket beside a zip.
Of course I can't take credit for this method, thanks in particular to @sewitwithdi for her blog about it, which you can find through her bio on Instagram. Also thanks to various vintage resources and patterns which refer to this method. There doesn't seem to be a recognised name for it, although I've seen it referrred to as "the couture method" more than once.
Here's the pdf if you want to download it: