Small holes can be closed using the same colour thread. Big holes or areas where the garment is worn thin can either be darned or patched.
Darning: You can use a darning egg to prevent the hole gathering together. Tack a thread around the hole and use it to sew in loose stitches so they don’t come undone. You then make a mini loom by sewing warp threads across the hole in one direction. You then weave across these threads.
Patching: Patches can be cut from worn-out garments (don’t throw them away), they can be knitted or crocheted. If you plan to cut the patches from old woollen garments, it can be a good idea to wash them at 40 degrees so they become a little felted. That will prevent them from unravelling. The patches can either be sewn on using a zigzag stitch on a sewing machine or sewn on by hand using tack stitches or buttonhole stitches. Make the patches quite big so they cover the entire worn-out area. There’s no need to camouflage the patches. Feel free to use contrasting colours and your imagination, and you can add some new character to the garment at the same time. Patched garments show that people look after things and don’t just use and throw them away.