In places where lot of strain is directed at garments the glueing method I did previously does not work, eventually the glue gives in and the hole in the trousers returns. The place I needed to fix was the butt of A's jeans.
These jeans will replace A's work jeans which are in much worse shape than these are. For the patch I cut out a piece of fabric and rounded the edges. Then I pinned the patch on to the back side trying to keep the fabric flat. Then I zigzaged the edges.
Then comes the most important part. I zigzag the whole patch area so it's filled. These stitches are what is keeping the fabric in one piece and keeping it strong.
From the right side it looks like this. If you choose a thread colour closer to the colour of the garment and make sure the fabric stays flat all the time the patch will be almost invisible.