I use lard as it is readily available here in the states. 1bar Ivory pure, 1lb lard, and 1 1/2 cup of water. Boil water and soap together, ad lard, mix and cool. As it cools give it a mix a few times. If it is too thick you can add water. I have a bucket of plaiting soap that is over 2 years old and has yet to go rancid.
Old plaiters would use sheep's fat from around the kidneys area (cleanest), render it down and strain it to remove impurities. Then add that to the soap and water. Also added a bit of kerosene to keep varmints from chewing the leather. I've used a few different scented oils like eucalyptus or Tea Tree oil.