Place the patch behind the hole so it sits against the inside of the leg. The patch needs to extend beyond the worn halo around the hole; stitches close to a fraying edge can tear out. This works best on woven school trousers with a localised tear and firm fabric around it.
Use a patch that stretches and recovers in the same direction as the knee fabric. A stiff woven backing on stretchy school trackies can make the knee feel tight and push the strain to the edge of the repair. Check the cloth first; some track pants use woven fabric and will need a different approach.
A knee that has gone shiny or thin but still has an unbroken surface can sometimes be reinforced from behind. The aim is to spread strain over sound fabric while keeping the front of the trousers smooth. Start by deciding whether you are looking at a real loss of cloth or just a visible mark.
Use this repair when the seam thread has broken and both fabric edges are intact. Backstitch along the existing seam line and overlap the good stitching at both ends. If either fabric edge has torn away, a line of stitches through the old holes will have little to hold; repair or reinforce the cloth first.
A pocket corner often needs a small reinforcement where the pocket opening joins the trousers. Look first for the original stitches pulling free. If the outer fabric has torn into a V or the pocket bag itself has split, repair that damage separately before reattaching the corner.
Assess the cloth around an inner-thigh tear before sewing. The area flexes and rubs as the wearer sits or walks. A neat seam can fail quickly if it is sewn into worn cloth or if tightening it makes the trousers too small.
A narrow binding can cover a frayed edge on a woven school cuff when the cuff body is sound and the opening still fits. The binding wraps the edge and takes the wear. A stretched knitted cuff needs the knit seam guide, and a tailored cuff with an affected placket may need more than an edge repair.
If only the edge of an iron-on patch has lifted and the fabric beneath is firm, sewing the edge down can be an option. Check the garment label and the patch instructions first. If you cannot establish that the fabric tolerates reheating, avoid trying to reactivate the glue.