Kaku-obi is most often used by men. It’s used under the formal hakama, and with normal yukata. It’s usually about 10 cm wide and 4 m long, so it’s even thinner than the half-width obi used by females. It can be made from many materials, and most of the ones used to make the obis for women can be used to make kaku-obi as well. Kaku-obi are often seen with Hakataori-style kimonos, but you don’t necessarily need to use one of them – because kaku-obi are easy to tie and lightweight, they can be easily combined with any kimono and so they are very popular among the buyers.
Please choose one that matches your kimono and yourself. You tie it with a kai no kuchi knot. This doesn’t mean that it has to be that way, but the finished shape became popular as it’s easy and hard to become loose. Other well-known methods are katabasami, ichimonji, and kandamusubi. Incidentally, the tip of kakuobi may have a tassel. Usually, it’s cut and the tip is inserted, but some people use it to decorate it. Either is fine.