Introduction to KOGIN
Kogin is a type of stitch that is somewhat similar to Norwegian 'smøyg' (drawn thread work) or counted stitch. Geometric patterns are created by covering areas of fabric with horizontal stitches, counting your way between loosely woven fabric and building up the pattern row by row.
Kogin was developed in Japan during the Edo period and is a variant of Sashiko. It is often referred to as Kogin-zashi. Coarse hemp or linen fabric was often the only fabric that poor farmers in Japan had access to between the 17th and 19th centuries. It was very cold and rough fabric and provided poor insulation even with many layers on top of each other.
By using counted stitch with thick cotton thread, a softer surface was created, more insulation was provided, and the garments became beautiful and could also be worn as dressier outfits. As they became worn, they were downgraded to everyday wear, with areas double-stitched, and sometimes the garment was dyed to camouflage discoloration from use. Traditionally, white thread was used on indigo-dyed fabric.
KOGIN KIT
I sell many kits for making new products in the online store. Although they are not materials for repair, they are great for learning a technique closely linked to meditative sewing, slow fashion, and you can use what you learn to reinforce and repair other woven garments or even knitted garments. Kogin thread and needle can be used for reinforcement, repair, darning, and decoration of clothes you already own.
MATERIALS
You don't need to buy a kit to try it out! The most important things are a fabric that is woven so that you can easily count the threads in the weave, a blunt needle so you can easily stitch between the threads in the fabric instead of through them, and a cotton thread. The advantage of the kogin needles from the online store is that they are longer than "tapestry" needles used for cross-stitch, for example. In cross-stitch, you only make one stitch at a time, and short needles are practical for that, whereas in kogin stitching, you make long horizontal stitches, and a long needle is useful so you don't have to pull the needle and thread up and down through the fabric so many times.
The thread is traditionally 100% cotton. Unlike embroidery thread, kogin thread is matte. You can also use other types of threads for kogin, such as embroidery thread, or darning thread for that matter. Kogin thread, like embroidery thread, consists of several strands that can be split apart so that the thread can be adapted to the thickness of your work. You can also use sashiko thread, but it cannot be split and must be used as is. Kogin thread is therefore also good for embroidery, darning, and repairs on clothes and textiles where you don't want shiny thread or wool thread and also want the ability to adjust the thickness.
You don't need an embroidery hoop, darning mushroom, or other equipment like that, but if you're using kogin-inspired techniques on knitted clothes, for example, it's a good idea to stretch them in an embroidery hoop and make sure the stitches are perfectly straight and parallel, as this will make it easier to count and get the work straight and even.

These videos show well how to sew. They are made by Olympus Japan, which is the manufacturer of the sashiko and kogin products in this online store.
Otherwise, threads and needles can be purchased separately and used for repair and decoration of clothes and textiles, with or without the kogin technique as a starting point.