What you need is a curved needle: it will allow you to sew from the top without ever needing to have access to the underside. They are cheap and widely available. When you use one to sew your patches to the pocket, you can use your hand to keep the pocket expanded, or even better put something small and solid in it to keep the two layers separated while you whipstitch the patch down from the top. Watch a video on how to use curved needles for repair.

Going by name, there are two kinds of curved needles sold: upholstery and embroidery. You absolutely want the embroidery kind: the upholstery needles are huge, made for much larger projects, and would make the fine work you are trying to do very difficult. Also, if you can pare down any hard coating on the underside of the patches around the edges without disturbing the patch itself, that will be helpful when you go to put it on.

If you get a thread that closely matches the beige of the patch it won’t matter how sloppy your stitches are, because they won’t be seen. But if you can, wash it and clean the grime off as much as possible before you do this, because it’s hard to match thread to dirty fabric. Also, when you are trying to match thread, sometimes value (light and dark) matters more than color. Because you’re sewing a lighter colored patch, if you can’t find an exact match, choose the closest one but go a shade lighter: the dark will stand out more on a lighter background.

If you have any problems or questions, feel free to hit me up. I’ve been doing my own hand and machine sewing for decades.