First, I cut the jeans down the side seams, and cropped them in length (orange lines).
I used that cut off piece as a template for my contrast denim (grey shape), adding a couple of inches along the straight side to upsize the jeans. I also added a couple inches on the top edge to fold over at the waist.
When I stitched the contrast pieces on, I completely messed up at hip level.
After a bit of swearing and a coffee, I chopped off the narrow straight bit from hip to waist, and stitched on separate pieces. I was on the verge of giving up as I now had two sloppy looking joins, so I used the cut off pieces from the leg length to fudge together a couple of half pockets to disguise the mess. I topstitched a lot (not perfectly, sadly).