My son is constantly wearing holes in his trousers. These are school PE trousers so kept the visible mend understated so he can still wear them for school. I used the visible mending technique called Boro stitch.
Firstly find a patch suitable for the area. For this one, I used an old t-shirt as it has the stretch in keeping with trouser material.
Stick in place with a glue stick which temporarily keeps in place so you can do stitching with confidence the patch will not move. Turn the right way out, so you can see the stitch design. I put a book or something in-between the trouser leg so you do not sewing the two sides together.
Do blanket stitch around the hole to secure the edges and stop them fraying.
Next, sew Boro stitch (photo 1 below), also know as running stitch around the hole or across the hole. In this design I did circles around the hole to secure the patch in place. Depending on your hole and design, you can do grid or any stitch pattern to secure the patch in place.
When finished the stitching, turn back inside out and trim patch material to near the edge of the stitching (photo 2 below), to reduce rubbing. Trousers good to go and look more stylish than when bought new!