Inspect while worn and flat
Look at the knee in the position where it normally bends, then lay the garment flat to mark the thinning. These views can show different edges of the repair.
Extend the patch beyond the weak area and avoid placing a hard corner directly on a fold line. A rounded or softly shaped patch may follow movement better.
Stabilise before decorating
Baste the patch through the stable layers and test a few stitches with the garment turned normally. Keep pockets and lining clear of the needle path.
Use moderate density and a thread that can tolerate repeated flexing. Very tight rows can reinforce the patch while making the knee feel stiff.
Check the repair in motion
Bend, sit and walk in the garment before the final edge finish. Watch for puckering, rubbing or a patch that shifts against the knee.
If the repair moves as one comfortable layer, finish the ends and record the care method. Future repairs can use the same patch margin and stitch spacing with confidence.