Insignia kit is embroided with golden metallic bullion on dark green cloth for Heer general peaked caps.
There were also metal insignias during WW2. Both metal and embroidered kits for generals' peaked-caps were equally common in the Heer.
How to sew the patch? Put the patch to the cap so that it sits well. Fasten the patch (for example, stitch it with a thread of noticeable color). Sew the patch by perimeter with small stitches over the edge, use a thread in the color of the backing. Set the needle 2-3 mm from the edge, otherwise the underlay will tear (shag), and the patch will fall off.