Assess the denim before you stitch
Wash the jeans according to their care label before repairing them, unless the fabric or garment instructions say otherwise. Clean cloth makes it easier to see the real shape of the wear and reduces the chance that later shrinkage will pull against the stitching.
Look beyond the hole itself. If the denim around it is thin, extend the repair into sound fabric so the stress is not concentrated on the old edge. Widespread thinning, stretched seams or areas that carry unusual loads may need a different repair or a specialist opinion.
Choose a patch, thread and needle that agree
Use a patch with a similar level of flexibility to the garment. It can match the denim for a quiet repair or contrast with it when you want the patch to be part of the design. The patch should reach into stable cloth on every side of the worn area.
Match the thread to the cloth and the look you want. Fine thread can sit neatly on closely woven denim; a thicker thread gives a clearer line when the patch and fabric can accept it without distortion. Choose a needle with an eye that does not abrade the thread and a length that feels controlled in the area you are repairing.
Prepare the layers and stitch across the weak area
Trim only loose fibres that will obstruct the work. Place the patch behind or over the worn section and baste or pin it so the layers cannot creep. Mark a simple grid, parallel rows or another restrained repeat that follows the shape of the repair rather than fighting it.
Run the needle through both layers, loading a few stitches when the fabric allows it. Keep the thread relaxed as it turns and cross beyond the old hole into sound denim. The purpose is to spread stress, not to pull the two layers into a rigid puckered ridge.
Inspect the reverse and care for the finished repair
Turn the jeans inside out before trimming the thread. Look for long loose jumps, tight channels or thread that has caught only the patch. Correct those areas while the repair is still easy to handle.
Wash the finished garment using the care instructions for the denim, patch and thread. Avoid assuming that a marker or temporary guide line can be ironed away; remove it according to its instructions before applying heat.