TOKYO — Nineteen years ago, when he began working at a factory that made watch straps, Kunitaka Kojima thought they were just something that came along with a timepiece. “I underestimated watch straps,” he said.

Now Mr. Kojima, 48, is an independent artisan who makes them by hand: “My wish is to make the most beautiful watch straps in the world, that will be used and admired for a long time.”

In 2012 he established his own brand, Galuchattail. Galuchat is a term for stingray and shark leathers (named — although misspelled — for Jean-Claude Galluchat, Louis XV’s master artisan, who introduced the exotic materials to France during the 18th century). As for the word “tail” in the brand name, Mr. Kojima said, it refers to “the stingray’s tail, which is sharp and edgy, how I want my designs to be.”

The straps, which start at 150,000 yen ($1,085) for the simplest version in crocodile, are designed to be taken apart for repairs. “If the material is damaged, you can replace it, or if the loop breaks, you can make a new loop and repair it over and over,” Mr. Kojima said.

His clientele includes watch collectors from around the world, independent watchmakers in Switzerland and Japan and microbrands such as Naoya Hida & Co., the Tokyo maker that outfitted its first watch, the Type 1C, with a hand-stitched brown leather strap from Galuchattail.

Mr. Hida said his initial visit to Mr. Kojima’s workshop was in 2018. “I was greatly impressed by his unparalleled commitment to quality and creativity,” the watchmaker said. “I immediately asked him to produce leather straps for our watches.”

Leatherwork is part of Mr. Kojima’s heritage. A great-grandfather was a tanner and, as a child, he played with small pieces of leather from his parents’ leather factory.

When he began working, he spent seven years as a tailor, but the cost of equipment deterred him from opening a shop. “I was looking for work where I could use my cutting and sewing skills, so I joined a watch strap factory in Tokyo,” he said, noting that he was part of a six-member team that produced 20,000 straps each month. “I worked 18-hour days, from 8 a.m. to 2 a.m., with one day off per month.”

The idea of a strap that could be repaired intrigued Mr. Kojima, and he set up a small atelier in Asakusa, a district on Tokyo’s eastern side that is known for the Senso-ji Buddhist temple, a major tourist attraction. Asakusa also was where Mr. Kojima was born and raised, although he now lives in Ebisu, on the city’s west side (“I keep a sleeping bag in the atelier,” he said, “for when I’m too focused on production and I can’t go back home after the last train runs out”).

In July, Mr. Kojima walked some guests, including myself, through the process of making his three strap styles: the 3-D Flex, a strap that matches the curve of the customer’s wrist; the 2-D Straight; and the Ready-Made, which, despite its name, also is customizable. He said that a standard Ready-Made strap could take three to five hours to complete, but that a custom-made one could take as long as six months.

Choose the Material

Mr. Kojima uses every kind of animal skin, but his signature choice is galuchat, which he buys from Thailand and Indonesia via Japanese leather wholesalers. He said he preferred pieces that are about 12 inches by 6 inches because the surface pebbling, also called denticles, is proportionally small and looks better than on larger skins. While he uses every bit of each piece, the portion with the white star-shaped mark found on a stingray’s spine commands a premium, raising the cost of a custom 3-D Flex galuchat strap to ¥250,000 from the usual ¥220,000 price. “The star mark is considered a good-luck charm that brings you towards the light,” Mr. Kojima said. Then he displayed a stingray hide covered in star marks, noting, “I waited five years to obtain it for a customer.” When that strap is finished, he said, it will cost around ¥500,000.

Polishing and Dyeing

Mr. Kojima dyes his own skins. “For galuchat, I first polish the denticles one by one, by hand, with a file to remove the small scratches, and polish them until they shine,” he said. Then he uses a brush to apply the dye, usually darkening the color as he works it toward the edges. “However, when dyed, the luster becomes a little cloudy, so I polish it manually again after the dye has dried,” he said. “And when you polish it, the color of galuchat will fade a little due to friction, so I dye it again.” He repeats the entire process, sometimes as many as 10 times, a process that could take as long as a month.

Fitting

For a custom order, Mr. Kojima measures the client’s wrist with a special gauge, an orange and black piece of plastic that envelops the wrist and looks like something made of Legos. “It’s the same as clothes’ tailoring, except for a wrist,” he said.

Designing and Cutting

Using the measurements, he draws the strap on graph paper and then, guided by the sketch, creates a pattern from a transparent sheet of plastic about half a millimeter thick. He then organizes his materials in a kind of sandwich: the galuchat; the core layer, a strong material called Cordura, made by the U.S. company Invista; and the lining, which usually is also a leather (Mr. Kojima recommends sharkskin because its uneven surface means the strap is unlikely to stick to the wearer’s wrist in hot weather). He places the pattern on top and then cuts through the layers using a leather knife made by a blacksmith who specializes in Japanese swords.

Shaving

“Before I assemble the watch strap, I shave the leather so the strap does not look untidy and I adjust the thickness of the leather,” Mr. Kojima said, displaying a kawasukiki shaving machine from the 1960s that he picked up secondhand. To demonstrate, he put a strip of leather against the machine’s spinning wheel, which rubbed it until Mr. Kojima decided it was thin enough to use.

Sewing and Assembling

As stingray leather is very hard, Mr. Kojima uses a small hand drill with a spiral bit to create holes and switches to a sharp straight bit to redrill each one. Then, with a bent needle and polyester thread coated with wax, he stitches through the galuchat and the core material (but not the lining because, he said, that might allow sweat or water to work its way up). “I sew by hand as I can perfectly control the pressure and speed, unlike using a machine,” he said. Finally, Mr. Kojima uses one of the five adhesives he keeps on hand to attach the lining.

Finishing

Mr. Kojima has his own method to seal a strap’s edges. First, he brushes dye on the raw edges and then applies a wax that he formulated. Once it is all completely dry, he heats a trowel over a lamp and gently rubs the warm tool over the edges to make the wax permeate the leather. He repeats the process 21 times to achieve a shiny and resistant edge, a process that generally takes about a week. “Painting the border means it’s easily repairable if damaged, and it’s also sweatproof and waterproof, as the water cannot enter,” he said. As for the buckle, Mr. Kojima will make one, or reuse one from an old strap provided by the customer.

The strap now is completed. With his patience, commitment to perfection and detailed explanations, I told Mr. Kojima he should be a teacher. “My dream is to open a school,” he said. “I’d like to teach young craftsmen to understand the essence of making a watch strap.”